u-boot:mc3: Add support for multiple devicetrees for u-boot and imx-boot

Adding new variable UBOOT_ADDITIONAL_DTB_NAMES to specify more then one
devicetree, use it in the u-boot and the imx-boot recipe to integrate
everything into one fit image.

imx-boot needs a patch, to use the addional devicetrees in the makefile
when writing the fit image.