tools/imximage: share DCD information via Kconfig

IMX based platforms can have the DCD table located on different
addresses due to differences in their memory maps (ie iMX7ULP).

This information is required by the user to sign the images for secure
boot so continue making it accessible via mkimage.

Signed-off-by: Jorge Ramirez-Ortiz <jorge@foundries.io>
Reviewed-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Stefano Babic <sbabic@denx.de>