We do not yet support USB on the Layerscape platforms, but when we do,
it's imaginable that we would want to export barebox_update targets over
Fastboot. Prepare for this by adding the layerscape images to those that
filetype_is_barebox_image returns true for.

Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>