We don't intend to ship any files in this directory, so there is no
risk of the directory getting too crowded over time. Drop the subdir
as this makes it easier for a outer build system to drop all the
needed firmwares into the correct directory.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>