airspeed: use _retries=1 for I2C retries once initialised

airspeed sensors often need to be on longer cables due to having to be
outside the prop wash.
-> Added APM makefiles to Gitignore.
-> Fixed PX4 makefile so it always copies the NuttX config files to PX4NuttX before rebuilding archives.
Merge remote-tracking branch 'upstream/master'
mixer: fixed stream handle leakage
px4iofirmware: fixed override when FMU dead 

this allows for immediate override if the FMU dies without requiring
that the override RC channel be in a particular position
px4io: added RC_CONFIG, SET_OVERRIDE_OK and CLEAR_OVERRIDE_OK ioctls

this allows for full setup of RC override without needing param_get()
to PX4 specific parameters
px4io: publish pwm values when STATUS_FLAGS_FMU_OK is not set

this allows testing of FMU failure behaviour in px4io by monitoring
the reported PWM output when the vehicle code stops sending
updates. Otherwise testing needs to be done with "px4io status" which
is very tedious.

With this change a GCS can monitor the PWM outputs from the failsafe
mixer using normal mavlink messages
px4io: fixed errno returns to be negative
px4io: added OVERRIDE_IMMEDIATE arming flag

this allows the flight code to choose whether FMU failure gives
immediate manual pilot control, or waits for the mode switch to go
past the override threshold
systemcmds: added reflect command for USB testing
px4io: only check SAFETY_OFF, not OUTPUTS_ARMED for allowing mixer changed

If we check OUTPUTS_ARMED then we can't update trim values and scaling
in flight, as there is no way to clear OUTPUTS_ARMED.

If safety is on then it should be perfectly safe to update the mixer
and RC config
Merge remote-tracking branch 'upstream/master'