AndroidKernel: Building dtb.img to support header version 2

The dtb.img is made by concatenating all the compiled dtbs
after the kernel is compiled. This dtb.img is then added to be
a part of the boot.img.

Change-Id: I6d8247cf5de99886f4514b5ce52711462aaf7e1c
Signed-off-by: Rahul Shahare <rshaha@codeaurora.org>