libdtovl: Add support for intra-overlay fragments
libdtovl: Add support for intra-overlay fragments

The kernel's dynamic overlay mechanism does not support fragments that
target other fragments, so modify dtoverlay_merge_overlay to make it
capable of partially merging an overlay with itself. After this pass
(indicated by passing in a NULL base_dtb pointer), all intra-overlay
fragments will have been applied and made dormant.

raspberrypi/userland,raspberrypi/userland,raspberrypi/userland