efi_loader: set partition GUID in device path for SIG_TYPE_GUID

Previously, the GPT device GUID was being used instead of the partition,
which was incorrect.

Signed-off-by: Alfonso Sánchez-Beato <alfonso.sanchez-beato@canonical.com>

Let EFI_LOADER select CONFIG_PARTITION_UUIDS.
Use log_warning() instead of printf() for warning.
Reviewed-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>