firmware: Restore support for built-in firmware

Commit 5620a0d1aac ("firmware: delete in-kernel firmware") removed the
entire firmware directory.  Unfortunately it thereby also removed the
support for built-in firmware.

This restores the ability to build firmware directly into the kernel by
pruning the original Makefile to the necessary minimum.  The default for
EXTRA_FIRMWARE_DIR is now the standard directory /lib/firmware/.

Fixes: 5620a0d1aac ("firmware: delete in-kernel firmware")
Signed-off-by: Markus Trippelsdorf <markus@trippelsdorf.de>
Acked-by: Greg K-H <gregkh@linuxfoundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
diff --git a/Makefile b/Makefile
index 7e2ca49..9f86816 100644
--- a/Makefile
+++ b/Makefile
@@ -562,7 +562,7 @@
 
 # Objects we will link into vmlinux / subdirs we need to visit
 init-y		:= init/
-drivers-y	:= drivers/ sound/
+drivers-y	:= drivers/ sound/ firmware/
 net-y		:= net/
 libs-y		:= lib/
 core-y		:= usr/