boot/uboot: allow to build multiple U-Boot images

boot/uboot: allow to build multiple U-Boot images

Sometimes it is desired to build multiple U-boot images. E.g. one to
save into flash memory and one for serial load. So far this was not
possible.

This change allows to select any combination of the target formats. They
are all copied to the image folder.

Signed-off-by: Petr Kulhavy <brain@jikos.cz>
[Thomas:
 - Handle the BR2_TARGET_UBOOT_FORMAT_BIN, which wasn't handled, and
   therefore u-boot.bin was not copied when
   BR2_TARGET_UBOOT_FORMAT_BIN=y.
 - Rename UBOOT_BIN to UBOOT_BINS, since it can now contain multiple
   values.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>