firmware: Correct dependency on CONFIG_EXTRA_FIRMWARE_DIR

When CONFIG_EXTRA_FIRMWARE_DIR gets changed, the filename in the .S file
(which uses .incbin to include the binary) needs to change. When we
renamed the BUILTIN_FIRMWARE_DIR option to EXTRA_FIRMWARE_DIR, we forgot
to update the manual dependency in firmware/Makefile, so it was
depending on a non-existent file in include/config/

Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>