the whole barebox with mbr and partition table were be signed by default.
change the signing to the executed image without signing the mbr,
partition table and header_gap by imx8mq
additional delete option full, from-dcdofs and skip-mbr

Signed-off-by: Maik Otto <m.otto@phytec.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>