Adds RallyPoint to Airfield

I see no reason why the Airfield should not have a RP. Perhaps it was just forgotten to update the yaml.
Definitely makes sense to have it.

Only one of many examples where this is useful: U got only one airfield and want to build several yaks. With the RP, you dont have to send each yak somewhere else on its own, just that another one can be build - the airfield is not blocked anymore.

Regards.
It is easier to understand the code if you don't hide the null check here in an extra bool.