firmware_loader: EXTRA_FIRMWARE does not support compressed files

Document in the firmware loader Kconfig help text that firmware image
file compression is not supported for builtin EXTRA_FIRMWARE files so
that someone does not waste time trying that.

Cc: Luis Chamberlain <mcgrof@kernel.org>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: "Rafael J. Wysocki" <rafael@kernel.org>
Acked-by: Luis Chamberlain <mcgrof@kernel.org>
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Link: https://lore.kernel.org/r/20220214222311.9758-1-rdunlap@infradead.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/base/firmware_loader/Kconfig

@@ -60,6 +60,8 @@ config EXTRA_FIRMWARE
 	  image since it combines both GPL and non-GPL work. You should
 	  consult a lawyer of your own before distributing such an image.
 
+	  NOTE: Compressed files are not supported in EXTRA_FIRMWARE.
+
 config EXTRA_FIRMWARE_DIR
 	string "Firmware blobs root directory"
 	depends on EXTRA_FIRMWARE != ""
@@ -169,6 +171,9 @@ config FW_LOADER_COMPRESS
 	  be compressed with either none or crc32 integrity check type (pass
 	  "-C crc32" option to xz command).
 
+	  Compressed firmware support does not apply to firmware images
+	  that are built into the kernel image (CONFIG_EXTRA_FIRMWARE).
+
 config FW_CACHE
 	bool "Enable firmware caching during suspend"
 	depends on PM_SLEEP