PX4: added eagletree I2C airspeed driver to the NuttX build

just needs a wrapper driver in APM now
AP_Param: fixed parameter save bug

This bug affected parameters where the defaults are overridden in the
object constructor. For example, a PID object may have a default value
for PID_D of 0.0, but have a constructor based default of 0.2. If the
user tries to set the value to exactly 0.0, then the set wouldn't happen,
as the value matches the value in the object default var_info[]
table. 

This change ensures we force a save to eeprom if the value is changing
from the current value, regardless of the var_info[] default.

Thanks to Tom Coyle for finding this bug!