DirectVolume: handle discs with multiple major numbers

CFlag: BOARD_VOLD_DISC_HAS_MULTIPLE_MAJORS

Allows Direct Volume to handle discs with multiple major numbers.
Needed on Samsung Galaxy S II and variants.

For example:
mmcblk0p1 - 179:0
mmcblk0p2 - 179:1
mmcblk0p3 - 179:2
mmcblk0p4 - 256:0
mmcblk0p5 - 256:1

Change-Id: If8f0f9d275e4642dd8e098817c993e07a5f1ada1