hi3518ev300_lite: build vfat/fat into kernel to free 4 KB of rootfs

[widgetii]

Summary

Nightly CI run 26667982165 failed for hi3518ev300_lite with rootfs.squashfs 5124 KB > 5120 KB cap (4 KB over). The lite variant has been parked at the cap for two weeks — the HISILICON_OPENSDK_TRIM_SP2308 finalize hook already drops three sensor .so files to keep this board buildable, and ~12 KB of accumulated drift since 2026-05-17 (a5614d9c) has now tipped it over.

This change flips two kernel symbols in hi3518ev300.generic.config from =m to =y:

-CONFIG_FAT_FS=m
+CONFIG_FAT_FS=y
-CONFIG_VFAT_FS=m
+CONFIG_VFAT_FS=y

Why these symbols specifically

general/overlay/etc/modules lists vfat as a boot-time autoload (loaded by general/overlay/etc/init.d/S35modules). vfat is the only .ko on this board that loads on every boot — so making it built-in is a pure layout shift, zero runtime cost. The other entry exfat belongs to the out-of-tree exfat-openipc package which is not enabled in this defconfig.

Every other =m symbol in this config (MAC80211, MT7601U, R8188EU, TUN, F2FS_FS, SCSI*, the CRYPTO_* family, etc.) is on-demand — USB-dongle insertion, WireGuard interface bring-up, USB-storage mount. Making any of those =y would burn unconditional kernel bytes for code that may never run on a given camera.

All NLS helpers vfat needs (NLS_CODEPAGE_437, NLS_ISO8859_1, NLS_ASCII) are already =y — no follow-up symbols needed.

Local build results

hi3518ev300_lite:

- uImage:          1873 → 1893 KB / 2048 KB  (155 KB headroom)
- rootfs.squashfs: 5124 → 5096 KB / 5120 KB  (24 KB headroom, was 4 KB over)

vfat.ko + fat.ko confirmed absent from output/target/lib/modules/; CONFIG_VFAT_FS=y / CONFIG_FAT_FS=y confirmed in built kernel .config; vfat_rename / namei_vfat.c strings present in vmlinux.

Cross-variant note

The same hi3518ev300.generic.config is referenced by hi3518ev300_ultimate_defconfig. Last passing CI had ultimate at uImage 1873/2048 (175 KB headroom) and rootfs.squashfs 7664/8192 (528 KB headroom); the same +20 KB kernel growth fits comfortably in both caps.

Test plan

• CI passes for hi3518ev300_lite
• CI passes for hi3518ev300_ultimate (shared kernel config)
• After merge, sysupgrade-flashed hi3518ev300_lite camera mounts a FAT-formatted SD card / USB stick without manual modprobe vfat

Follow-up (separate PR)

The lite variant remains within ~25 KB of the cap with the temporary TRIM_SP2308 block still removing SP2308 + SC500AI + GC5603. The underlying drift (most likely hisilicon-opensdk bump #2156 or unpinned majestic master) warrants a bisect + compaction pass so those sensors can be restored.

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