scripts: Makefile: add support to use external DTC

Add support to use external DTC to compile device tree blobs.

Change-Id: I268d2332d5328c3b0050b35c96f91e8ef9e1f6f8
Signed-off-by: Shashank Mittal <mittals@codeaurora.org>
Signed-off-by: Puja Gupta <pujag@codeaurora.org>
[eberman@codeaurora.org: Resolve merge conflicts]
Signed-off-by: Elliot Berman <eberman@codeaurora.org>