tools: imx8image: add possibility to skip dcd

tools: imx8image: add possibility to skip dcd

Currently U-Boot always adds DCD Image to boot container.
On imx8qxp SoC it is possible to init RAM from within SCFW,
and adding a DCD image type to the boot container in this case
breaks booting (No debug output anymore from SCFW! Nor any
output from SPL), so we need to configure a dcd skip somehow.

This patch adds a new imx8image_cmd entry CMD_DCD_SKIP and a
new entry in imximage.cfg "DCD_SKIP". If set to "true"
no DCD image type will be added to the container.

Signed-off-by: Heiko Schocher <hs@denx.de>
Reviewed-by: Peng Fan <peng.fan@nxp.com>