KERNEL_DEVICETREE may contain more than one DTB, the first one being the
default one. Therefore, let's split on space first, to get the first DTB
before stripping the directory name from it.
This doesn't add support for creating multiple labels for each DTB in
KERNEL_DEVICETREE.
Reviewed-by: Trevor Woerner <twoerner@gmail.com>
Fixes: 3b51866f22 ("remove /boot partition")
Fixes: 13316b7968 ("KERNEL_DEVICETREE: 32-bit re-org")
Signed-off-by: Quentin Schulz <quentin.schulz@cherry.de>