[fix][lpc21] LPC + xbee_api in rotorcraft
workaround for lpc: sys_time_usleep only seems to work after a enableIRQ()
closes #352
[nps] if NPS_JS_MODE_AXIS is defined: use it as axis for the mode channel instead of buttons
add fraser jsbsim quadrotor model with front/back motors turning CCW, left/right CW