meas_airspeed: don't take the aboslute value
airspeed_calibration: stop talking about Pa and and hashtags
Revert "airspeed_calibration: stop talking about Pa and and hashtags"

This reverts commit c6fb75f66fa94444d07056f271980e3f05008f94.
airspeed_calibration: stop talking about Pa and and hashtags (now the correct files)
meas_airspeed: don't reset the filter below 0
vfr_hud mavlink msg: use baro alt

The vfr_hud message demands the AMSL altitude and not the wgs84
altitude. Use the baro altitude for now. This can be changed to an
output of the position estimator later.
Remove absolute pressure field as its not useful and confusing anywary
Merge pull request #1308 from PX4/airspeedfabs

No absolute value of airspeed
Merge pull request #1309 from PX4/vfrhudbaroalt

vfr_hud mavlink msg: use baro alt
Merge branch 'master' of github.com:PX4/Firmware into ekf_varweight