AHRS: changed default RP and YAW gain to 0.3

this reduces the impact of hard acceleration on takeoff, and reduces
the impact of GPS lag

Note that this doesn't affect copters, as they override to 0.1
AHRS: removed AHRS_BARO_USE option

this option has caused users too much trouble. The vertical velocity
is too noisy from the baro
Copter: remove ahrs.set_barometer()
Plane: removed ahrs.set_barometer()
Plane: removed old HDNG2RLL PID control

not used with L1
autotest: precreate buildlogs directory
AP_L1_Control: change turn_distance() to be min of wp_radius and L1 distance

this gives less surprising behaviour for users
HAL_PX4: if we have lost RC input then set channel 3 to 900

this allows failsafe to kick in
Plane: allow for negative WP_LOITER_RAD for conter-clockwise loiter

this follows the same convention as missions
Plane : AP_L1_Control : Fix wrong way turn behaviour on loiter entry
L1_Control: cleanup some unused code and variables