wic: add option to specify the diskid

wic: add option to specify the diskid

This adds a feature to specify the disk ID when creating a disk with
the wic tool. This is useful when using the DOS partition scheme and
booting with root=PARTUUID=<partuuid>. In DOS partitions, the partition
ID is <diskid>-<partition-number>, so it makes sense to let the user
define the disk ID.

You can specify it in the kickstart file using the --diskid argument
to the bootloader command. The value can be given in decimal or
hexadecimal format (e.g. 3735928559 or 0xdeadbeef). If omitted, the
previous behaviour does not change.

Signed-off-by: Steffen Greber <sgreber@lilafast.org>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>
Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>