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: Idb6d3a4970a9d9e775968a18594532b62a93d434
Signed-off-by: Rahul Shahare <rshaha@codeaurora.org>