stub: add StubDevicePartUUID/StubImageIdentifier

stub: add StubDevicePartUUID/StubImageIdentifier

These variables closely mirror the existing
LoaderDevicePartUUID/LoaderImageIdentifier variables. But the Stub…
variables indicate the location of the stub/UKI (i.e. of systemd-stub),
while the Loader… variables indicate the location of the boot loader
(i.e. of systemd-boot). (Except of course, there is no boot loader used,
in which case both sets point to the stub/UKI, as a special case).

This actually matters, as we support that sd-boot runs off the ESP,
while a UKI then runs off XBOOTLDR, i.e. two distinct partitions.