libfdt will assume a writable fdt overlay blob has been passed in, so make ours compatible to allow easier review when we try to drop libfdt into place. overlay from the calling context is writable, making it safe to simply rip out everything related to copying the overlay blob in fdt_overlay_apply.
I note here that we still have problems: fdt_overlay_apply, both our version and libfdt's, may fail and have already clobbered the base fdt to some extent. Future work will make sure we don't apply a potentially bogus fdt, instead discarding the base fdt if we had an error.