PX4: enable ets_airspeed driver if connected
AP_Airspeed: support ARSPD_PIN option for choosing source

this gives us support for arbitrary analog pins for the airspeed
sensor, plus support for the EagleTree airspeed driver on PX4
Plane: use the new airspeed driver interface

the choice of analog source is now in the airspeed driver
autotest: tweak throttle and pitch control for auto-landing
Copter: bug fix for take-off when throttle is zero

Verify_takeoff was always returning false if throttle was zero.  The
pilot should be able to move their throttle back to zero immediately
after initiating the mission
Copter: point nose at landing point if specified
RCMapper: simple mapping object for primary input channels

allows arbitrary mapping of first 4 channels
Copter: integrate draft RCMapper
RC_Channel: removed global rc_ch[] array

hide it as a object static instead
AP_Mount: use RC_Channel::rc_channel() instead of rc_ch[]
Plane: use RC_Channel::rc_channel() instead of rc_ch[]
Rover: use RC_Channel::rc_channel()
Copter: removed use of rc_ch[] global array
Plane: allow channel mapping of first 4 channels

this makes it easier to support DSM and SBUS radios
AP_Mount: make the code a bit easier to read
RC_Channel: added output_trim() and read() functions

these make using rcmap in the plane code easier
Plane: fixed remaining places where primary channel ordering was assumed

should now use rcmap for all inputs
AP_RCMapper: improved docs for RCMAP_*
Plane: rename old channel_* globals

prevent the old "g." names from being used
Plane: make it possible to change control channel ordering without reboot

this makes configuration a bit easier