AP_HAL: added voltage_latest() interface

this will be used for reading the 5V rail
AP_HAL_AVR: support voltage_latest()
HAL_PX4: support all analog input pins, with scaling

this allows voltage/current/Vcc sensing on FMUv2
Rover: fixed Vcc
Copter: fixed Vcc
Plane: fixed Vcc
AP_ADC_AnalogSource: fixed APM1 build
Rover: update battery pin docs
Copter: allow Vcc checks on Pixhawk
Copter: update battery pin docs
Plane: update battery pin docs