cmake: Omit default priority ELF symbol.

[casaroli]

Summary

nuttx_add_application() translates SCHED_PRIORITY_DEFAULT into a zero-valued nx_priority linker symbol for CMake-built ELF applications. Priority zero is invalid for a runnable application task; on assertion-enabled targets, launching such an application triggers the scheduler priority assertion.

Do not emit nx_priority when an application selects SCHED_PRIORITY_DEFAULT. The ELF loader already uses its scheduler default when the symbol is absent. Explicit numeric priorities continue to be emitted unchanged.

This is the CMake companion to apache/nuttx-apps#3667, which makes the corresponding correction for Make-built application ELFs. Together they replace the need for the broader ELF-loader workaround proposed in #19537.

Impact

• Feature/API/ABI: No new feature or API/ABI change.
• Users: No user action required. CMake-built applications using SCHED_PRIORITY_DEFAULT receive the existing scheduler default instead of an invalid priority-zero ELF attribute.
• Build: CMake kernel/loadable-ELF link metadata changes only. Applications with explicit numeric priorities are unchanged.
• Hardware/architecture: None; the CMake helper is architecture-independent.
• Documentation: No update required; behavior now matches the existing ELF-loader default semantics.
• Security: No security impact.
• Compatibility: Improves compatibility with the loader's default-priority behavior. It does not alter explicitly configured numeric priorities.

Testing

Host: Fedora Linux x86_64; CMake 4.3.0; Ninja 1.13.2; xPack riscv-none-elf-gcc 14.2.0.

Target configuration: rv-virt:knsh64 (RISC-V 64-bit kernel ELF build).

Commands:

cmake -S nuttx -B cmake-knsh64 -G Ninja -DBOARD_CONFIG=rv-virt:knsh64
ninja -C cmake-knsh64 -t commands ELF_ostest
ninja -C cmake-knsh64 -t commands ELF_getprime
./tools/checkpatch.sh -f cmake/nuttx_add_application.cmake

After this change, the generated ostest link command contains no nx_priority definition:

riscv-none-elf-ld ... -o .../bin_debug/ostest

The explicit numeric-priority regression check is retained:

riscv-none-elf-ld ... --defsym nx_priority=50 ... -o .../bin_debug/getprime

A generated-build-tree search found no --defsym nx_priority=0. ./tools/checkpatch.sh -f cmake/nuttx_add_application.cmake and git diff --check pass.

The full CMake/Ninja rv-virt:knsh64 build could not complete on this host because CMake invokes this xPack toolchain's riscv-none-elf-ld with its default 32-bit emulation while the target objects are 64-bit:

target emulation `elf64-littleriscv' does not match `elf32-littleriscv'

This is unrelated to the priority change. No physical-hardware runtime validation is available in this workspace, so this PR is marked draft pending that validation.

PR verification self-check

• This PR introduces one focused CMake build-system correction.
• All required description fields are updated.
• The commit follows the Contributing Guidelines, including Assisted-by and human sign-off trailers.
• This PR is a work in progress pending hardware build/runtime validation.
• This PR is ready for review and can be safely merged.

[casaroli]

Companion PR: apache/nuttx-apps#3667 fixes the same default-priority ELF encoding in the Make build path. This PR covers the CMake path.

[casaroli]

@xiaoxiang781216

[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

[anchao]

LGTM