efi_loader: use short-form DP for load options

The GUID of partitions is sufficient for identification and will stay
constant in the lifetime of a boot option. The preceding path of the
device-path may change due to changes in the enumeration of devices.
Therefore it is preferable to use the short-form of device-paths in load
options. Adjust the 'efidebug boot add' command accordingly.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>