tecs.cpp: tecs initialisation fixes (#411)
tecs.cpp: tecs initialisation fixes (#411)

- zero thrust integrator on initialisation, we already have a cruise throttle
variable
- if initialising from in_air condition set last throttle setpoint to
cruise throttle (_in_air ? throttle_cruise : 0.0f);
PX4/ecl,PX4/ecl,PX4/ecl,PX4/ecl,PX4/ecl