Makefile: Modify the script to allow for a different output dir

This change adds the ability to have a different output directory for the
binaries generated by external dtc. This change is used by the kernel
platform project to have precompiled binaries available for use.

Change-Id: I5d5ca942da76f1c53b5b14999e3a5d8a92f9195d
Signed-off-by: Siddharth Gupta <sidgup@codeaurora.org>