libdiskconfig: set MBR signature when creating MBR

When formatting a blank SD card, it needs to set MBR signature when creating MBR.
If not, the 'format' operation will fail.

Change-Id: I860731243797d4da83f370af52822536983009ee
Signed-off-by: Cylen Yao <cylen.yao@mediatek.com>