Plane: Doc. update for ALT_HOLD_RTL with Rally Points.
Plane: Introducing FENCE_RETALT parameter.
Plane: removed some compile warnings from rally.pde
AP_Math: fixed build of vectorN class on PX4
AP_HAL: added HAL_CPU_CLASS define for selecting algorithms

this will make it easier to select the appropiate level of algorithm
for a CPU
AP_Baro: use HAL_CPU_CLASS in baro driver
DataFlash: use HAL_OS_POSIX_IO
HAL_Linux: fixed build
Plane: default LOG_BITMASK to 32767 on non-APM
HAL_PX4: removed code which drops throttle to 900 on loss of RC

rely on vehicle code checking for last data coming in
px4: enable dual telemetry by default on FMUv1

this makes us compatible with wiring diagrams from PX4 site
Plane: fixed redundent logging
HAL_PX4: fixed RCInput race condition

the number of channels could be zero with thread switch at the wrong
time
Git: ignore .elf files
Copter: BugFix DO_SET_ROI returns Lat,Lon,Alt to GCS
DataFlash: only fsync every 10 seconds

prevents too much work in SITL
DataFlash: add EnableWrites method

This method allows temporarily suspending writes to the dataflash which
will be used in ArduCopter to ensure no messages are written while the
copter is disarmed
Copter: suspend logging while disarmed
Copter: remove armed check before logging
OptFlow: shrink lib by removing unused functions

Saves 46bytes of RAM and 1k of flash