Support cloning to a loopback device

With some additional scripting around rpi-clone, this allows cloning
into a loopback-mounted image file.

This is a first step towards fixing geerlingguy:#9.

Author: matthijskooijman

rpi-clone

@@ -1075,6 +1075,9 @@ then
 		else
 			assumed_fstab_edit=0
 		fi
+	elif [[ $dst_disk == *"loop"* ]]
+	then
+		dst_part_base=${dst_disk}p
 	else
 		qecho $"
   Target disk $dst_disk ends with a digit so may be a partition.