Switch airframe corrrectly

[manfer33]

To use any other airframe and load its correct mixer is necessary to edit the line 34 of px4.config

For example, if u want to load the hexa mixer: linux_pwm_out start -m ROMFS/px4fmu_common/mixers/hexa_x.main.mix

I think this information should be in airframe_reference section

Verified in commit dc8caea from master

[bkueng]

Since this is RPi specific (or rather a Linux-target problem, but it's different for other linux boards or it does not apply, such as for Bebop), I'd add a section to https://dev.px4.io/en/setup/building_px4.html#raspberry-pi-23-boards. Also note that the airframe page is auto-generated.