HAL_PX4: prevent excessive writes on startup from blocking

this could cause copter on PX4 to hang on startup
Copter: fixed baro-only preflight cal

this was causing a full level, which mucked up the INS calibration
HAL_AVR: switch back to 500kHz SPI for MPU6k until we understand the issues

this is for new plane release. We need for analysis before we can
re-enable high speed SPI
Plane: improved perf monitoring with SCHED_DEBUG
Rover: fixed performance monitoring

now the same as plane
Plane: ready for 2.75 release
Copter: fixed HIL build
BoardLEDs: all leds off during init
BattMonitor: add @Increment to CAPACITY description
Plane: add @Increment to battery failsafe descriptions
Copter: add @Increment to battery failsafe descriptions
Copter: add ANGLE_RATE_MAX param

Limits the maximum rotation rate requested by the angle controller which
is used in stabilize, loiter, rtl and auto flight modes
AP_Airspeed: fixed I2C semaphore handling for I2C airspeed

this affects MS4525DO on APM2
Copter: crash checker

Crash is determined to have happened when the copter is 20deg more than
the ANGLE_MAX parameter continuously for more than 2 seconds
Not activated when in ACRO mode or while flipping
Copter: add alt and throttle checks to crash detector
Copter: remove some commented out send_text code
Copter: correct @Range of MOT_TCRV_MAXPCT parameter
Copter: disable some aux channels on hexa and octas

Resolves issue #324
RC_Channel: formatting
Copter: rc10, rc11 available even without MOUNT