distro_bootcmd: Set distro_bootpart_uuid for block devices

The assignment of block device nodes in Linux is not deterministic by
default, i.e. a newly added eMMC controller or other block device can
change the assignment of /dev/mmcblkN (or other block device node like
e.g. /dev/sdXy) and prevent the system from picking the correct block
device for root filesystem in case the root filesystem is specified on
kernel command line using 'root=/dev/mmcblkNpM' (or 'root=/dev/sdXy'
etc.).

One way out is to derive PARTUUID in U-Boot, which is unique identifier
of a partition, and pass that as root=PARTUUID=<partuuid> to Linux via
kernel command line. Linux would then find the partition using PARTUUID,
no matter on which block device the partition resides and which node was
assigned to that block device.

Derive the PARTUUID before scanning for extlinux presence and assign it
into distro_bootpart_uuid environment variable, which can then be used
in extlinux.conf kernel command line specifier.

Note that it is not possible to do this in scan_dev_for_extlinux script
because this script is called from scan_dev_for_boot script, which is
called for both block devices as well as UBI volumes, and we can not
derive PARTUUID for UBI volumes.

Signed-off-by: Marek Vasut <marex@denx.de>