flyingf3: add support for ms5611 baro on rcflyer shield

This is entirely untested since I don't have an rcflyer shield.

This might be sufficient to make this work for some users though.

We may not want to merge this since (I think) not all rcflyer shields actually ship with this baro on it.  We may need sub-options in hwsettings for each shield to say exactly which components are populated.

We should also figure out a sane way to separate the shield-specific init from the core init before this gets entirely out of hand.