make: put buildtest goals into an own file

`Makefile.include` is too long as is.
make: delete everything in buildtest beforehand
make: supply `-j` on `buildtest`

We support `make -j` for a while now. `make buildtest` takes ages to
finish. So just supply `-j` on buildtest.
Merge pull request #1169 from Kijewski/buildtest-minus-j

make: supply `-j` on `buildtest`