ARM: software serial port integration (Tx only)

- be careful charging functionality not tested
- work corectly with baud rate 19200 and lover
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- TMR2 is used for Tx timing
- ADC for temp is disabled when Tx soft serial is active (maybe timing problem for other functionality)
- some logic are added for enable/disable some funcionality when eternal temp or Tx serial is active (probably not enough)
actually I need ENABLE_EXT_TEMP_AND_UART_COMMON_OUTPUT
for the atmega32 version where I have one charger with "common" uart and temp (imax B6)
and another (GTPower A6) with a separate connector, but they use the same uart.

I also would like to have both uarts that could be selected in "settings" (but this is for a far future release).
Maybe the simplest solution would be to create a simple Serial.h file that looks like this:

inline void Serial::initialize() {
#ifdef ....   //for now
  TxSoftSerial::initialize();
#else 
  HardwareSerial::initialize();
#endif
}
... (and so on)