Latest nuttx 7.22+ with PX4 contrib for stm32 f4 I2C hang.

  Prep for Nuttx Upgrade - still needs i2c fix

   board_adc_init()    - initalise the ADC HW once.
   board_adc_sample()  - read a given channel dn

   Internal functions are public functions that should realy only
   be called by the board config.

  Updated to latest upstream with PX4 contrib for STM32 I2C that
  fixes an hang in driver.

   A battery is considered connected when a) V > BOARD_ADC_OPEN_CIRCUIT_V
   and if BOARD_ADC_OPEN_CIRCUIT_V > BOARD_VALID_UV connected
   is further qualifed by the Valid signal.

   Due to higher bias, V open circuit on the ADC is high with a 1M
   termination. This override the default connected threshold on
   V5 HW. Revist once lowe termination is chosen.

   Based on the R values on the LTC4411 the px4fmu-v4pro has a
   UV of 4.01 Volts.

   BOARD_ADC_OPEN_CIRCUIT_V is the voltage present on an ADC due
   to the bias current on the terminition resistor.

   BOARD_VALID_UV is the under voltage min set by resistors on a
   board's Power selector.

   A battery is considered connected when the Voltage measures is
   greater than BOARD_ADC_OPEN_CIRCUIT_V.

   In the case where BOARD_ADC_OPEN_CIRCUIT_V is greater then
   BOARD_VALID_UV we can use the HW to qualify connected.

   Connected is assumed when the battery voltages is greater than
   a predefined level.

   The data sheet for the F4, F7 indicate a maximum of 36 Mhz
   in the 2.4-3.3 volt Soc operating range. This change sets
   the clock based on the STM32_PCLK2_FREQUENCY.

   Upstream change on F7 CONFIG_STM32F7_FLASH_PREFETCH and
   CONFIG_ARMV7M_ITCM are now contorled by
   CONFIG_STM32F7_FLASH_ART_ACCELERATOR