docs: sysctl/kernel: document firmware_config

Based on the firmware fallback mechanisms documentation and the
implementation in drivers/base/firmware_loader/fallback.c.

Signed-off-by: Stephen Kitt <steve@sk2.org>
Link: https://lore.kernel.org/r/20200429205757.8677-2-steve@sk2.org


Signed-off-by: Jonathan Corbet <corbet@lwn.net>