tinkerboard: disable x265 and document platform-specific Mesa/LLVM failure path

[Copilot]

The issue requested opting for option 1: disable x265 only on tinkerboard, and explain why the same build break was not reproduced on other platforms. This change scopes x265 off to tinkerboard while leaving shared/global package selection unchanged elsewhere.

• Scope x265 disablement to tinkerboard only

  • Added a tinkerboard defconfig override to unset x265, instead of changing shared Buildroot.config.
  • Other platforms continue to inherit the global x265 setting.

• Platform-specific failure analysis

  • Recent tinkerboard failures were in mesa3d (gallivm/lp_bld_misc.cpp) with LLVM 22 API usage (StringMapIterator), not in x265.
  • The failing compile path is tied to tinkerboard’s ARM32 + Panfrost/Mesa combination; other targets use different architecture/driver combinations and do not hit the same codepath.

# buildroot-external/configs/tinkerboard.config
# BR2_PACKAGE_X265 is not set