Adds USART6/AUX2 support also to Jumper T16 & T18. TX and RX lines are accessible via internal FrSky Bluetooth module pads on the mainboard. The USART6/AUX2 serial port functionality is usable more generic on these radios as well and not limited to Bluetooth only.
E.g. radio GPS is supported after this change (as are further serial port modes).
Only if firmware is explicitly built with -DBLUETOOTH=YES, USART6 will be dedicated to Bluetooth (as is the case also e.g. with TX16S).
