[KERNEL][D18] Add new class to test devicetree overlay combinations during build

This class allows a defined list of overlays to be applied to base devicetrees during the yocto build. This should bringup logical errors like missing nodes, wrong node names ... more early compared to on-target tests. Also the coverage of combinations can easily be increased.

This class is inheritet by the seco-imx kernel.

The D18 config is updated to enable the combination test to combine the D18 base devicetree with either the B79 or the MV baseboard and all other overlays on top.

MODV-76