gk7202v300: add ultimate variant + ship rtl8188fufw blob from the FU package

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Summary

Closes #2153 (no gk7202v300 ultimate build + RTL8188FTV not working) and fixes #1906 (HI3518EV200 ultimate same RTL8188FU firmware-blob symptom) as a side effect.

• rtl8188fu-openipc: the rtl8188fufw.bin firmware blob was sitting inside rtl8188eus-openipc/ and was only installed when BR2_PACKAGE_RTL8188EUS_OPENIPC=y, which no production defconfig sets. Boards that enabled BR2_PACKAGE_RTL8188FU_OPENIPC=y (e.g. hi3518ev200_ultimate) therefore compiled 8188fu.ko but never shipped /lib/firmware/rtlwifi/rtl8188fufw.bin, so the driver couldn't bind. Moved the blob into the FU package and install it from a POST_INSTALL hook there.
• gk7202v300_ultimate_defconfig (new): cloned from gk7205v200_ultimate_defconfig (same SoC family) with the gk7202v300.generic.config kernel config and SOC_MODEL=gk7202v300. Adds BR2_PACKAGE_RTL8188FU_OPENIPC=y and BR2_PACKAGE_LINUX_FIRMWARE_OPENIPC_RTL_8188EU=y so the 16 MB ultimate image natively drives the reporter's RTL8188FTV (USB 0bda:f179) via the existing /etc/wireless/usb rtl8188fu-generic shim.
• CI: added gk7202v300_ultimate to the ultimate matrix in .github/workflows/build.yml.

Local-build verification (make BOARD=gk7202v300_ultimate)

- uImage:         1890 KB / 2048 KB
- rootfs.squashfs:7064 KB / 8192 KB
- Build time:     04:05

unsquashfs -l rootfs.squashfs.gk7202v300 | grep -E '8188fu|rtl8188fufw|wpa_supplicant':

squashfs-root/etc/wpa_supplicant.conf
squashfs-root/lib/firmware/rtlwifi/rtl8188fufw.bin
squashfs-root/lib/modules/4.9.37/extra/8188fu.ko
squashfs-root/usr/sbin/wpa_supplicant

Both the driver and the firmware blob land in the squashfs at the expected paths. Image fits comfortably within the NOR-16 MB caps.

Out of scope

• gk7202v300_lite was deliberately not modified — adding the FU driver+blob to the 8 MB rootfs (already close to the 5120 KB cap) needs a separate size-budget audit. The reporter's hardware ships with 16 MB NOR, so the ultimate path covers their case.
• On-device smoke test (flash → fw_setenv wlandev rtl8188fu-generic → reboot → wlan0 associates) should be done by the issue reporter or a lab board after merge.
• Regression rebuild of hi3518ev200_ultimate was not run locally (Buildroot mirror was flaky enough that one full build took multiple retries with hand-seeded dl/ tarballs); the CI matrix will exercise it on this PR.

Test plan

• CI matrix builds gk7202v300_ultimate cleanly
• CI matrix hi3518ev200_ultimate continues to pass (no regression from the blob relocation)
• Reporter (gk7202v300: no ultimate build available + RTL8188FTV (0bda:f179) WiFi not working in lite build #2153) flashes the produced image to BM002-IPC-GK7202-PCB-V10 + RTL8188FTV and confirms wlan0 comes up with wlandev=rtl8188fu-generic

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