Linux packaging

Fixes linux packaging and adds a new "linux_standalone" target.
> Perhaps @solidgoldbomb can comment, but it seems like a better way to pass this variable would be
> include $(ROOT_DIR)/make/tools.mk
> in package/Makefile. It seems strange to see that as the only variable passed in.

That is easier said than done,  ROOT_DIR is defined system-id.mk so you would have to include that, but in that file ROOT_DIR is defined based on CURDIR so that gets messed up, even if you work around that other issues come to play.
Bottom line is that this is an urgent fix since linux jenkins builds are currently broken, we can fix style later IMHO.
#1228 shows that linux recipe currently calls 'make -j2 GCS_BUILD_CONF=release GCS_QMAKE_OPTS="CONFIG+=SDL CONFIG+=KML" gcs' after this is merged calling 'make -j2 GCS_BUILD_CONF=release GCS_QMAKE_OPTS="CONFIG+=SDL CONFIG+=KML" linux_standalone
' instead should make things work.