init: Add partition_uuid to Uevent

As of commit upstream Linux kernel commit 74f4a8dc0dd8 ("block: add
partition uuid into uevent as "PARTUUID""), it's easy to include the
partition UUID in the Uevent structure. Add it in so that other parts
of the init code can make decisions based on the partition UUID.

If this code is run on older kernels we'll never see the partition
UUID and it will be left blank.

Bug: 316324155
Test: Run w/ a newer kernel and see partition_uuid populated.
Change-Id: I48a52aa006c05023f7f1cc5cc0ab5c1f1ec37455