Currently this just calls the zynq_mkimage script to stamp in the header
checksum. Can be extended to a proper multi-image build later on.

This requires a PBL to be build, but as the only supported Zynq board
already selects the PBL option in the defconfig there is no big change
to the previous status

Signed-off-by: Lucas Stach <dev@lynxeye.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>