base-files: export whole disk of fitblk backing partition

base-files: export whole disk of fitblk backing partition

After commit ad62247800 ("base-files: improve lib/upgrade/common.sh")
behavior of export_bootdevice has been made consistent in such way that
always the whole disk device is exported (as that was the case already
when matching via UUID) rather than the partition device.
Do the same for the device holding the fitblk backing partition.

Fixes: 5992f976b3 ("base-files: recognize bootdevice on devices using fitblk")
Signed-off-by: Daniel Golle <daniel@makrotopia.org>