RC_Channel: don't change trim if radio in is zero
Plane: don't change trims if radio_in is zero
HAL_PX4: use write() to /dev/pwm_output for servo output

this lowest the cost of PWM output a lot, but relies on the new I2C
based IO firmware
HAL_PX4: added support for /fs/microsd/APM/nostart

prevents APM startup
HAL_PX4: startup in low priority to fix CLI

this ensures that tight sensor loops in setup() can run without
sleeping