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/firmware/.gitignore b/firmware/.gitignore
new file mode 100644
index 0000000..d9c6901
--- /dev/null
+++ b/firmware/.gitignore
@@ -0,0 +1,6 @@
+*.gen.S
+*.fw
+*.bin
+*.csp
+*.dsp
+ihex2fw