[Feature request] Add `FW_LOADER_COMPRESS` to kernel config

[srcshelton]

RPi kernels are (now?) configured with XZ-compressed modules 👍

Could CONFIG_FW_LOADER_COMPRESS and CONFIG_FW_LOADER_COMPRESS_XZ also be set to allow XZ-compressed firmware too, please?

This would allow me to use a unified compressed firmware package across multiple platforms whilst still using the stock Raspberry Pi kernel, and shouldn't be a high-impact change since this compression scheme is already used for modules.

Thanks for your consideration!