cmd: mvebu: bubt: verify A38x target device type

Ensure that the device to which an image is being written includes
header information indicating boot support for the destination
device.

This is derived from the support in the SolidRun master-a38x vendor
fork.

Signed-off-by: Joel Johnson <mrjoel@lixil.net>
Reviewed-by: Stefan Roese <sr@denx.de>