Add factory reset feature

The `factory-reset` command provides a way to completely wipe the data
partition, thus erasing any local configuration and state.  The command
itself simply enables a special systemd service unit that is activated
during the shutdown process.  This unit runs a script, after all
filesystems, except rootfs, have been unmmounted.  It then erases the
signature of the filesystem on the data partition, so it will appear
blank the next time the system boots.  This will trigger the
`init-storage` process, to create a new filesystem on the partition.

overlay/usr/lib/systemd/system/factory-reset.service

@@ -0,0 +1,13 @@
+[Unit]
+Description=Perform a factory reset
+DefaultDependencies=no
+After=umount.target
+Before=systemd-reboot.service
+
+[Service]
+Type=oneshot
+ExecStart=/usr/libexec/factory-reset
+StandardOutput=kmsg+console
+
+[Install]
+WantedBy=final.target