fdtoverlay: A tool that applies overlays

Since libfdt support overlay application on FDT blobs, provide
a command line tool that applies an arbitrary number of
overlays, one after another to a base fdt blob and output
the result in the given file.

Signed-off-by: Pantelis Antoniou <pantelis.antoniou@konsulko.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>