systemd-bootdisk.wks: use PARTUUID

systemd-bootdisk.wks: use PARTUUID

Root device name in systemd-bootdisk.wks is 'sda'. This can cause
images, produced using this wks to refuse booting if real device
name is not 'sda'. For example, when booting MinnowBoard MAX from
MicroSD card the boot process stucks with this message on the boot
console output: Waiting for root device /dev/sda2...
This happens because real device name of MicroSD card on this device
is mmcblk1.

Used --use-uuid option for root partition. This should make
wic to put partiion UUID instead of device name into kernel command
line.

[YOCTO #10485]

(From OE-Core rev: 5b73d5f484cc844affe91ec19d881d42e187f30c)

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