lsm303d: use DRDY pins to automatically reschedule measurements

this prevents double reads of sensor data, and missing samples from
the accel
lsm303d: added 'lsm303d regdump' command

useful for diagnosing issues
lsm303d: define some more register addresses
lsm303d: print more registers in "lsm303d regdump"
SPI: added set_frequency() API

this allows the bus speed to be changed on the fly by device
drivers. This is needed for the MPU6000
mpu6000: change bus speed based on registers being accessed

this ensures we follow the datasheet requirement of 1MHz for general
registers and up to 20MHz for sensor and int status registers
px4io: moved blue heartbeat LED to main loop

this allows us to tell if the main loop is running by looking for a
blinking blue LED
lsm303d: added detailed logging of accels on extremes

this will log accel values and registers to /fs/microsd/lsm303d.log if
any extreme values are seen
lsm303d: fixed TEMP_H register define
lsm303d: log mag regs too
lsm303d: always log first ARB and REG values
lsm303d: show regs at both high and low bus speed on error
lsm303d: zero-fill register reads
FMUv2: added support for MPU6000 on v2.4 board
FMUv2: added define for MPU DRDY pin
FMUv2: set MPU6000 CS as initially de-selected
l3gd20: added I2C disable based on method from ST engineering support
lsm303d: added I2C disable based on method from ST engineering support
lsm303d: get cleaner logic traces by gathering all regs more regularly
lsm303d: cleanup logic traces by pre-zeroing all transfers