stub: don't override LoaderDevicePartUUID EFI var

stub: don't override LoaderDevicePartUUID EFI var

We document and all our code assumes that LoaderDevicePartUUID is
initialized to the ESP's UUID. Let's hence not override the variable if
it is already set, in order to not confuse userspace if the kernel's EFI
image is run from a different partition than the ESP.

This matches behaviour for all other variables set by the EFI stub, in
particular the closely related LoaderImageIdentifier variable.