change serial configuration : gps can be used with softserial

what is possible  :
* msp/mainport = uart1, uart2
* serialrx = uart2 
* telemtry = msp/mainport  , soft1, soft2
* gps = uart1, uart2, soft1, soft2   (if port is allreadyuse , gps is not start)

user can not lock himself out from the cli.

the port used by each function is configured in cli .

SERIALPORT_SOFT_1 = 0
SERIALPORT_SOFT_2 = 1
SERIALPORT_UART_1 = 2
SERIALPORT_UART_2 = 3

tested with : sepktrum sat , usb connected and gps over softserial 2 , ie :

feature serialrx
set gps_port = 1
set msp_port = 2
set gps_baudrate = 4
set serial_baudrate = 115200
set softserial_baudrate = 19200

![gpsserial](https://cloud.githubusercontent.com/assets/1732355/2898727/b88b7446-d5a5-11e3-85cf-ebb07a42ecb0.png)