tools: imx8image: set dcd_skip to true

To B0[+] chips, dcd_skip needs to be true. For A0 chip, it needs
to be false, however A0 chip is no longer being supported anymore.
Considering we are moving code from imx-mkimage to uboot mkimage,
to make sure we not introduce some surprise, we still keep dcd_skip
code there.

Signed-off-by: Peng Fan <peng.fan@nxp.com>