u-boot: support merging .cfg files for UBOOT_CONFIG

u-boot: support merging .cfg files for UBOOT_CONFIG

U-boot recipe supports .cfg files in SRC_URI, but they would be merged
to .config during do_configure only when UBOOT_MACHINE is set, we
should also support merging .cfg files for UBOOT_CONFIG.

Signed-off-by: Max Krummenacher <max.krummenacher@toradex.com>
Signed-off-by: Ming Liu <ming.liu@toradex.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit ce3de00d41791fa5e9557c5e93a99fbe368311f5)
Signed-off-by: Steve Sakoman <steve@sakoman.com>