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
HAL_Linux: add PCA9685 based RCOutput for Navio
HAL_Linux: add GPIO driver for Raspberry Pi
HAL_Linux: Initialize i2c before rcoutput (required by i2c pwm chips for initialization)
HAL_Linux: separate RCInput for PRU and Navio
hal_ap_linux: Make PRU constants private to reduce #define namespace pollution.

Forthcoming Zynq port AP_HAL_Linux uses similar constants with different
values - move these as private member constants to avoid collisions.

Signed-off-by: John Williams <john@whelanwilliams.net>
HAL_Linux: fixed linux build
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
AP_NavEKF: Split getAccelBias into getAccelZBias and getIMU1Weighting