Added RSSI injection on a RC channel from receiver.
Compatible with analog RSSI readings
http://www.multiwii.com/forum/viewtopic.php?f=8&t=5530

git-svn-id: http://multiwii.googlecode.com/svn/trunk/MultiWii_shared@1698 02679b44-d973-9852-f2fa-63770883b36c