Adds support to add mbn header to u-boot.bin as mbn header is needed by
sbl of all boards with ipq806x soc. A python script at tools/mkheader.py
is included which creates u-boot.mbn from u-boot.bin.

Change-Id: I070d82d9af0ed772cd4970b8fde152a8e3068823
Signed-off-by: Gokul Sriram Palanisamy <gpalan@codeaurora.org>