Improve RS232 hw handling

[stianeklund]

This issue isn't strictly related to this code base as it will need some hardware solutions. It's still relevant IMO to do some of the code changes to facilitate this.

The RS232 port on the radio is already in use, current hardware implementation only listens serial data, no tx is hooked up. If AI mode is not AI2 then the antenna switch will not track any frequency changes initiated via CAT. This isn't an issue if the change is initiated from the serial / USB side.

Need to create a HW solution for better RS232 handling (improving on the current RS232 sniffer setup):

• Use multiplexer to switch between serial lines (multiplexer --> level shifter)
• MAX339 & MAX3221 combination or similar can probably achieve this?

Detect whether or not RRC is on (check RTS/CTS perhaps?):

• If RS232 from radio is active and AI2 don't do anything
• If RS232 from radio is not active and AI mode is not AI2 --> we can use the tx line.
• If RS232 from radio is active but not AI2 use smart logic to temporarily switch outputs so we can query the radio for a second or so, e.g. FA; IF; wait a second or so then switch lines back.