firmware: Add CONFIG_EXTRA_FIRMWARE option

This allows arbitrary firmware files to be included in the static kernel
where the firmware loader can find them without requiring userspace to
be alive.

(Updated and CONFIG_EXTRA_FIRMWARE_DIR added with lots of help from
Johannes Berg).

Signed-off-by: David Woodhouse <dwmw2@infradead.org>
Signed-off-by: Johannes Berg <johannes@sipsolutions.net>