Fix ros-core and switch to -j1 since -j8 seems to get stuck alot on downloading sources.

Fixed ros-core port, now behaves the same as the ros-groovy port.

Additionally:
'Port install' timed out a lot of times due to limited network resources. Changing the init flag to -j1 solved this problem for me.

Signed-off-by: Ruben Smits <ruben@intermodalics.eu>