Add support for DTB overlays to ubldr:
- Add handler for fdt_overlays variable to ubldr.
"fdt_overlays" is coma-separated list of overlay files. ubldr would try to load files one by one from /boot/dtb/ and right before final DTB fixup it would try to apply them to current FDT blob. Variable can be either u-boot env variable or loader env variable.