... and some strange reformatting due to astyle...

This will initialize those structs with zero in all fields not set
and all fields set will only be change once to the final value not
wasting CPU time zeroing it.

This will guarantee that no non-unitialized structs will have
a trash value on from_external causing it to be sent to the
MAVLink channel without need it.

The previous approach was checking system id and component id but it
will not work in 100% of cases as external devices can send MAVLink
message with the right system id but with broadcast component id.

No need to keep this struct as global or alive while looping.

Lets save a few bytes using the right data types.

   Define BOARD_NUMBER_I2C_BUSES and BOARD_I2C_BUS_CLOCK_INIT
   for I2C1-4

   Used the board overrideable vlaues to define the _bus_clocks
   array and initalize it.

   Define the default I2C buss frequncies that are backward compatible
   with the existing code. While allowing it the defaults to be overridden
   by a board config.

   Based on the legacy STM32 code, the I2C buss numbering starts at 1.
   The bus frequency is stored in a 0 based array. If px4_i2cbus_initialize
   returns a valid device, then the _bus-1 will act as the index to the
   busses frequency.

   A board may define BOARD_NUMBER_I2C_BUSES - the number of I2C busses
   it supports* and BOARD_I2C_BUS_CLOCK_INIT to initalize the bus
   clocks for a given busses.

   BOARD_NUMBER_I2C_BUSES - the number of busses including the *highest
                            number bus. If the board has 2 I2C
                            busses I2C1 and I2C3 BOARD_NUMBER_I2C_BUSES
                            would be set to 3

   BOARD_I2C_BUS_CLOCK_INIT - Initalization for the bus frequencies
                              by bus. A call init, with a frequency
                              less then the value used for the
                              Initalization will result in the device
                              not starting becuase the buss runs too
                              fast for it.

  PX4_NUMBER_I2C_BUSES number of busses that the HW can support
  not all of them my be usesd. I.E. The STM32F427 has 3 I2C
  busses but only I2C1 and I2C3 are used.

This uses the TEMP1 & TEMP2 fields, which probably were used for something
else initially. However this implementation matches with OpenTX and APM.

Also add the vehicle_gps_position & flight mode information