arch/arm: add Realtek RTL8721Dx and RTL8720F (Ameba IOT) support

[raulcxw]

Summary

Add NuttX support for two Realtek Ameba dual-core WHC (WiFi Host
Controller) SoCs and their evaluation boards:

SoC	Board	AP core (NuttX)	NP core (WiFi)
RTL8721Dx	pke8721daf	km4 (Cortex-M33)	km0
RTL8720F	rtl8720f_evb	km4tz (Cortex-M33)	km4ns

On these parts NuttX runs on the application (AP) core; the WiFi
MAC/PHY firmware runs on the network (NP) core, and the two cores talk
over the vendor IPC. The host WiFi stack is driven through a WHC glue layer.

What this PR adds:

• arch/arm/src/rtl8721dx/ and arch/arm/src/rtl8720f/ chip support, plus a
  shared arch/arm/src/common/ameba/ layer (IPC, WHC WiFi host glue, flash
  MTD, LOGUART console, SDK fetch/build tooling).
• boards/arm/rtl8721dx/pke8721daf/ and boards/arm/rtl8720f/rtl8720f_evb/
  with an nsh defconfig each.
• Platform/board documentation under Documentation/platforms/arm/.

Working features (both boards): NSH over LOGUART, WiFi STA scan + connect,
WiFi SoftAP, littlefs on the on-chip flash (MTD), and AP↔NP IPC.

How the dual-core / vendor-SDK integration works

• Out-of-tree, pinned SDK — no vendored blobs. The build auto-fetches a
  pinned commit of the public ameba-rtos SDK (shallow clone) into
  arch/arm/src/common/ameba/, provisions its toolchain/venv, and compiles the
  needed register-layer (fwlib) and WHC glue sources from SDK source into
  per-board archives. The SDK tree is kept pristine (zero patches) — it
  natively supports the single-core / boot build entry points we use.
• NuttX owns the image2 entry. arch/arm/src/<chip>/ameba_app_start.c is a
  NuttX-licensed adaptation of the SDK app_start() that performs the
  OS-independent silicon init and then calls NuttX main(). No SDK source file
  is modified.
• NP/boot built from the same pinned SDK every build, fed NuttX's own
  disassembly so the SDK's "noused" WiFi-API generator keeps exactly the host
  APIs NuttX references (avoids an NP deadlock). This guarantees the two cores
  never drift.
• All vendor build artifacts live in a git-ignored prebuilt/ staging dir;
  the SDK kconfig snapshots the packaging step needs are regenerated from the
  pinned SDK on every build (never committed).

Impact

• New architecture/boards only — adds new files under arch/arm/src/...
  and boards/arm/...; the only edits to existing files are the additive
  registration entries in arch/arm/Kconfig and boards/Kconfig. No existing
  board or architecture is affected.
• Build system: make only. The board CMakeLists.txt intentionally emits
  a clear "CMake build not yet supported" FATAL_ERROR for these boards.
• Network: standard NuttX netdev / wireless ioctls; no core net changes.
• Documentation: new pages only.
• Security: none (no changes to common code paths).

Testing

Built and run on real hardware for both boards.

Build (host: Ubuntu, arm-none-eabi from the SDK-provisioned toolchain):

./tools/configure.sh pke8721daf:nsh   && make    # -> app.bin, boot.bin, km0_image2_all.bin
./tools/configure.sh rtl8720f_evb:nsh && make    # -> app.bin, boot.bin, km4ns_image2_all.bin

The same configs are built in CI on a clean container using the official
apache-nuttx-ci-linux image (fresh SDK fetch from the pinned commit), and the
resulting firmware was flashed and verified on hardware:

• NSH prompt over LOGUART, basic shell commands.
• wapi/iwconfig WiFi scan and connect to a WPA2 AP; DHCP lease and
  ping over the air.
• SoftAP brought up; a client associates and obtains a DHCP address.
• littlefs mounted on the flash data partition (read/write/persist across
  reboot).

Notes for reviewers

• The vendor SDK is auto-fetched (pinned), not vendored — there are no
  binary blobs in the tree, and prebuilt/ is git-ignored.
• nxstyle emits "Mixed case identifier" on vendor SDK symbol names
  (e.g. Img2EntryFun0, SYSTIMER_Init) referenced from the adaptation files.
  These are the SDK's public API spellings and cannot be renamed.
• RTL8720F links the SDK lib_rom.a with --whole-archive (its ROM symbols
  must all be present); this is required by the silicon's ROM layout.
• A fork-only GitHub Actions workflow used to produce/verify the CI firmware is
  not part of this PR.

[github-actions]

MemBrowse Memory Report

No memory changes detected for:

• arduino-mega2560
• esp32-devkitc
• hifive1-revb
• mirtoo
• qemu-armv8a
• qemu-intel64
• rx65n-rsk2mb
• s698pm-dkit
• stm32-nucleo-f103rb

[acassis]

@raulcxw nice contribution! Kudos!!

Please fix this CI issue:

./env.sh: line 223: /github/workspace/sources/nuttx/arch/arm/src/common/ameba/ameba-rtos/.venv/bin/activate: No such file or directory
/github/workspace/sources/nuttx/arch/arm/src/common/ameba/ameba-rtos/.venv/bin/python: No module named pip
ameba_setup_env.sh: SDK venv (/github/workspace/sources/nuttx/arch/arm/src/common/ameba/ameba-rtos/.venv) is missing json5/click.
  Most likely the venv was not created properly -- install the python
  venv module (e.g. 'apt install python3-venv') so the SDK env.sh can
  build .venv with pip, then retry.  (A flaky PyPI can also cause it.)
make: *** [tools/Unix.mk:529: prebuild] Error 1
make: Target 'all' not remade because of errors.

Please remove reference to this .venv python

[acassis]

@raulcxw please fix this issue:

chip/ameba_wifi_init.c: In function 'rtl8721dx_wifi_initialize':
Error: chip/ameba_wifi_init.c:161:3: error: implicit declaration of function 'up_enable_irq'; did you mean 'up_enable_icache'? [-Werror=implicit-function-declaration]
  161 |   up_enable_irq(RTL8721DX_IRQ_IPC_KM4);
      |   ^~~~~~~~~~~~~
      |   up_enable_icache
``

[acassis]

OK, I will merge since the reported error:

Configuration/Tool: stm32l0538-disco/nsh,CONFIG_ARM_TOOLCHAIN_GNU_EABI
2026-07-01 10:33:30
------------------------------------------------------------------------------------
  Cleaning...
  Configuring...
  Disabling CONFIG_ARM_TOOLCHAIN_GNU_EABI
  Enabling CONFIG_ARM_TOOLCHAIN_GNU_EABI
  Building NuttX...
arm-none-eabi-ld: /github/workspace/sources/nuttx/nuttx section `.data' will not fit in region `flash'
arm-none-eabi-ld: region `flash' overflowed by 20 bytes
make[1]: *** [Makefile:230: nuttx] Error 1
make: *** [tools/Unix.mk:569: nuttx] Error 2
make: Target 'all' not remade because of errors.
/github/workspace/sources/nuttx/tools/testbuild.sh: line 397: /github/workspace/sources/nuttx/../nuttx/nuttx.manifest: No such file or directory
  [1/1] Normalize stm32l0538-disco/nsh

is not related to this PR.