wic: rewrite MBR disk identifier

wic: rewrite MBR disk identifier

Disk identifier created by parted doesn't match the one we generated
and used in bootloader config. We need to rewrite it to make our image
bootable.

Modified involved API and data structures to access previously
generated disk identifiers after MBR is initialized. Written disk
identifiers to MBR.

(From OE-Core rev: 221d3bdd6e0ab8a4d25e2c96bd976cbec4e76681)

Signed-off-by: Ed Bartosh <ed.bartosh@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>