fix bug that prevented /sim/sceneryloaded from ever becoming "true" if

fix bug that prevented /sim/sceneryloaded from ever becoming "true" if
/sim/screneryloaded-override was "true". At least one subsystem (od_gauge)
waits for /sim/sceneryloaded to bypass CPU intensive code until the scenery
is up. This broke e.g. the RTT-radar when using /sim/screneryloaded-override

--prop:sim/sceneryloaded-override=true has the effect that fgfs
doesn't show the splash screen until the scenery is loaded, but shows
the OTW view (scenery/aircraft) at the earliest possible moment. This is
useful for developers who often need to run fgfs only to check some minor
detail, while not caring about stuttering caused by scenery loading.