Handling already mounted persistence backing media correctly.

I.e. treat the case where a backing media already had been mounted in
the expected place like a success (by printing the mount dir). This
doesn't fix any bug in live-boot, but it will come in handy for the
(soon-to-come) script for activating custom persistence in user-space.

While we're at it, document this function.

scripts/boot/9990-misc-helpers.sh

@@ -714,6 +714,11 @@ try_mount ()
 	fi
 }
 
+# Try to mount $device to the place expected by live-boot. If $device
+# is already mounted somewhere, move it to the expected place. If
+# we're only probing $device (to check if it has custom persistence)
+# $probe should be set, which suppresses warnings upon failure. On
+# success, print the mount point for $device.
 mount_persistence_media ()
 {
 	local device=${1}
@@ -751,8 +756,12 @@ mount_persistence_media ()
 			rmdir "${backing}"
 			return 1
 		fi
+	else
+		# This means that $device has already been mounted on
+		# the place expected by live-boot, so we're done.
+		echo ${backing}
+		return 0
 	fi
-	return 0
 }
 
 close_persistence_media ()