This commit provides a recipe for a core image to start roscore, and
also tunes the README.

In the current state, the catkin recipe does not work on the target system.
We want to keep the native compilation of ros groovy working, even if our
current development is focusing on the cross-compilation.
To make the native compilation of ros groovy more stable, we go back to the
previous state where even catkin was compiled on the target system.
Hence, this commit reverts commit 2c8274b7
to obtain that previous state.

The conflicts in the files of this commit were manually resolved.

improving dependencies in the python recipes; simplifying core-image-ros after dependencies are improved

adding dateutil to the image, as i do not know why RDEPENDS does not ensure that dateutil exists in the rootfs

tried to add python-vcstools via dependency in python-rosinstall but it does not do and then added it the core-image instead; tuned layout and order in core-image