fdtdump: add a --scan option

Often times, fdts get embedded in other larger files.  Rather than force
people to `dd` the blob out themselves, make the fdtdump file smarter.

It can now scan the blob looking for the fdt magic.  Once locate, it does
a little validation on the main struct to make sure we didn't hit random
binary data.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>