[WIP]: add board specific airframe mechanism

[dagar]

TODO: how to handle airframe (SYS_AUTOSTART) numbers

[hamishwillee]

Before this goes in, will need to clarify what it means for docs

• Does it mean different airframe reference? http://docs.px4.io/master/en/airframes/airframe_reference.html
• Where are they defined
• What do we need to say here? https://dev.px4.io/master/en/airframes/adding_a_new_frame.html

[stale]

This issue has been automatically marked as stale because it has not had recent activity. Thank you for your contributions.