RSSI monitoring on CC3D

RSSI monitoring is implemented on CC3D.
This has needed remapping the RC_input connector as the Stm32f103 PB5 line (previous unused RC4 pin) has no ADC converter. The RC_input connector is now : 
RC1   : ground
RC2   : + 5V
RC3   : PPM
RC4   : Tx Softserial port
RC5   : Rx Softserial port
RC6   : Current
RC7   : Vbattery
RC8   : RSSI

As a side effect, Current and RSSI use the same PA1 and PB1 lines than other boards, resulting in a "Cleaner" coding.

A PPM->RSSI conditionner is needed (see diagram in pull request #150).