Add builtin firmware support ...

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
[andrew.smirnov@gmail.com: Add dummy.o in case directory is empty]
Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>