Conditionalize the inclusion of local copies of bsd.*.mk. They will only
be included if they exist in devel/portmk/Mk; otherwise, the fallback file
from ports/Mk will be used. This will allow us to remove local copies of
files once their changes are merged back into ports/Mk, so that not every
single change in ports/Mk needs to have a tracking commit made back into
devel/portmk/Mk. In theory this should allow us to integrate changes a
little more quickly.