boot-directdisk: allow specifying custom MBR disk signature

boot-directdisk: allow specifying custom MBR disk signature

This introduces a DISK_SIGNATURE variable that allows controlling the
32-bit MBR disk signature. By default it is set to an automatically
generated disk signature but it may by overridden in the image recipe
by setting DISK_SIGNATURE to a 8 digit hex string.

This DISK_SIGNATURE variable can also be used in the image recipe to
specify the root by UUID using:
SYSLINUX_ROOT = "root=PARTUUID=${DISK_SIGNATURE}-02"

Specifying the root by UUID allows the kernel to locate the root
filesystem even if the device name changes (e.g. /dev/hda2, /dev/hdb2 or
/dev/sdb2 instead of /dev/sda2) due to differences in hardware
configuration.

(From OE-Core rev: 4382a419b4c90312d22aa55ff535b45bcf704716)

Signed-off-by: Jonathan Liu <net147@gmail.com>
Acked-by: Darren hart <dvhart@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>