layerscape: add multiple-configuration generator for FIT images

layerscape: add multiple-configuration generator for FIT images

This is required for the Traverse LS1043 family, we generate a FIT image
that works on all boards across the family. This is done by creating
multiple configurations that select the right DTB for the board.

The bootloader on these boards is configured to boot like this:
bootm $kernel_load#ls1043s
bootm $kernel_load#ls1043v

This is based on earlier work by Jason Wu for Zynq:
https://lists.openwrt.org/pipermail/openwrt-devel/2016-March/040460.html

Modified to add FDT load addresses and multiple configurations with DTB.

Signed-off-by: Mathew McBride <matt@traverse.com.au>