mavlink HIL with normal autostart scripts

Work on seamless HIL switching. User will not need to set special `SYS_AUTOSTART`, but only set HIL mode with mavlink command. `mavlink` app will start sending `HIL_CONTROLS` stream and listening `HIL_SENSORS`/`HIL_GPS`/`HIL_etc` streams.

`mavlink` app already done, need to implement HIL mode support in `gps` and maybe other apps.