Move dtc and libfdt sources from arch/powerpc/boot to scripts/dtc

The powerpc kernel always requires an Open Firmware like device tree
to supply device information.  On systems without OF, this comes from
a flattened device tree blob.  This blob is usually generated by dtc,
a tool which compiles a text description of the device tree into the
flattened format used by the kernel.  Sometimes, the bootwrapper makes
small changes to the pre-compiled device tree blob (e.g. filling in
the size of RAM).  To do this it uses the libfdt library.

Because these are only used on powerpc, the code for both these tools
is included under arch/powerpc/boot (these were imported and are
periodically updated from the upstream dtc tree).

However, the microblaze architecture, currently being prepared for
merging to mainline also uses dtc to produce device tree blobs.  A few
other archs have also mentioned some interest in using dtc.
Therefore, this patch moves dtc and libfdt from arch/powerpc into
scripts, where it can be used by any architecture.

The vast bulk of this patch is a literal move, the rest is adjusting
the various Makefiles to use dtc and libfdt correctly from their new
locations.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>