HMC583 and MS5611: Make sure they return error codes on init

Previously the HMC5883 locked up and the MS5611 didn't return an
error when the sensors were not found.
Freedom: Indicate panic codes for different hardware failures

Base on the number of pulses between longer pauses:
1 - flash chip failure
2 - mpu6000 failure
3 - hmc5883 failure
4 - ms5611 failure
PIOS I2C: Add a function to check the bus is clear

This is mostly useful for startup to verify that something isn't blocking
the I2C bus.  Have example use in Freedom pios_board.c.
Freedom V2: Update the pin mappings
Freedom Overo Interface: Use SPI Mode 0
Overo logger: Wait a few seconds to enable the SPI dev

Something about the initial power on of the overo causes some
bit shifts from the flight controller and makes it unreliable.
Delaying enabling the spi peripheral ensures the overo is in
a known stable state first.
Freedom: Add comment describing the error codes
Merge pull request #280 from peabody124/freedom_v2

Support for Freedom v2