PiOS ADC: Added new full_range_value function to upper driver structure
PiOS ADC: Added new full_range_value function to all lower drivers

Added function to:
F1,F3,F4 and PCF8591 drivers
PiOS ADC: Added new PIOS_ADC_GetChannelAdjusted function to upper driver

This function divides the raw value by the full range value and
multiplies it by 3.3 (max voltage).
The returned value is therefore the true voltage as seen in the input
pin.
Modules Battery:Used PIOS_ADC_GetChannelAdjusted and modified calculation formula.

Now the calibration value is divided by the value read. According the
what I have seen most vendors advertise their sensors as X/volt and
X/amp so to facilitate the required configuration by the user and let
him use that value directly we need to divide the read value by the
vendor referred value.
PiOS F3 ADC:Slowed down ADC clock to reduce CPU usage.

This reduces CPU usage from 58 to 50% on flyingF3. This shouldn't have
any flight impact since the ADC isn't being used with sensors which need
high update rates.
PiOS ADC:Renamed PIOS_ADC_GetChannel and PIOS_ADC_GetChannelAdjusted

PIOS_ADC_GetChannel to PIOS_ADC_GetChannelRaw
PIOS_ADC_GetChannelAdjusted to PIOS_ADC_GetChannelVolt
Flight:Added VREF_PLUS definition to targets pios_board.h file

Defined as 3.3 on all targets except discovery board based targets where
it was defined as 3.
TODO check all targets schematics and check if this is true.
PiOS ADC:Lower drivers refactored PIOS_INTERNAL_ADC_Full_Range_Value

Now called PIOS_INTERNAL_ADC_LSB_Voltage, uses VREF_PLUS value from
board file and returns the voltage corresponding to LSB.
PiOS ADC:Driver device structure, renamed full_range_value to lsb_voltage

Update lower drivers with new function pointer name.
PiOS ADC:Upper driver refactored PIOS_ADC_GetChannelVolt to use lsb_voltage
Merge pull request #1029 from guilhermito/simplify_ADC_config

Simplify voltage and current sensor configuration