Merge pull request #525 baro_abi

Baro event handling using ABI interface

Main benefits:

less crappy handler functions in main.c files
possibility to use multiple sources of sensors (integrated to the board or not)
the filters don't need to know who is sending the raw values (not completely true with the old alt_float filter yet)

the pressure are now standardized in Pascal and the standard atmosphere model is used

The INS_BARO_SENS is hence not needed anymore.
onboard baros are automatically available in fixedwing firmware as well (use same code as rotorcraft now)

disable onboard baro with <configure name="USE_BARO_BOARD" value="FALSE"/>
Some stuff to do before using: find the default conversion factor to convert ADC values to Pascal
(mostly older boards/baro, recent digital sensors can output pressure in Pascal already)

Also all baro_board implementations use the same BARO_BOARD_SENDER_ID,
since there can only be one baro_board at the same time and this provides a good default for INS_BARO_ID.
So if you want to use an onboard baro the INS_BARO_ID is already ok (also if you have additional baro modules).
To use a baro module for INS: <define name="INS_BARO_ID" value="BARO_x_SENDER_ID"/>

closes #525

conf/airframes/ardrone2_raw.xml

@@ -109,7 +109,6 @@
   </section>
 
   <section name="INS" prefix="INS_">
-    <define name="BARO_SENS" value="22.4" integer="16"/>
   </section>
 
   <section name="STABILIZATION_RATE" prefix="STABILIZATION_RATE_">