utils-systemd: provide enumeration of units

This enumeration is based on the filesystem,
units are listed from the configured directory.
This is necessarily because not all units can be
loaded by systemd but all can be started.

Change-Id: Ic4d1331c8c54f5bbaa747ff8084da2b0c5a65c55
Signed-off-by: José Bollo <jose.bollo@iot.bzh>