PX4: conditionally build auth command and libraries

this makes it possible to build with upstream master
PX4: add TESTS_MATHLIB_DISABLE define

this helps building with upstream master
PX4: fixed startup log msg location
autotest: added developer build support

this creates firmware builds for developers in the devbuild directory
autotest: added link to devbuild tree
AP_HAL_Linux: [Bugfix] prevent memory acces violation in process_dsm_pulse

When a S.Bus signal is fed into the rcin-Pin AruPlane on the BBB get's a
Segmentation fault. This patch prevents the memory acces to dsm_state.bytes[16] outside
of the array bounds. The patch should be reviewed by sombody who knows the DSM protocol,
as i don't.
scripts: fixed devrelease permissions
AP_AHRS : Tidy up logic used to enable synthetic sideslip fusion
DataFlash : Change to more descriptive EKF logging names
AP_NavEKF : Remove ineffective numerical divergence checks
AP_NavEKF: clean up unused variable lastDivergeTime_ms
Replay : Remove unused numerical divergence metric
Replay: clean up unused variables deltaGyroBias and divergeRate
DataFlash : Remove unused numerical divergence metric
AP_NavEKF: Set prevDelAng after using it
DataFlash: fix bug in ekf gyro bias logging