Weak symbols have problems on cygwin. These are only defined and used in
SITL so no point having an empty definition here, which would cause a
redefinition error if not weak.
Keeping the declaration in AP_HAL itself is not the best, but it works
and avoids significant surgery to AP_HAL_SITL to move their declarations
and usages into a better place.
Thomas Watson
bugobliterator
Andrew Tridgell
Peter Barker