Installation of ros-base on debian stretch

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I want to install ros-base pkg(likely Melodic) on an Beaglebone Black Debian Stretch Image. link text

Unfortunately I am confused now:

Firstly there are no melodic nor kinetic packages available after updating and

 sudo sh -c 'echo "deb http://packages.ros.org/ros/ubuntu $(lsb_release -sc) main" > /etc/apt/sources.list.d/ros-latest.list'

Instead I can install ros-core, ros-base I tried:

sudo apt-get install build-essential libboost-all-dev device-tree-compiler schedtool screen chrony cgroup-lite
sudo apt-get install python-scipy ipython python-qt4
sudo apt-get install dirmngr
sudo apt-get install cmake python-empy python-nose libgtest-dev
sudo apt-get install python-rosdep python-rosinstall-generator python-wstool python-rosinstall build-essential
sudo apt install python-dynamic-reconfigure python-catkin-pkg python-rospkg-modules python-roslib python-rospkg python-rosbag python-roslaunch python-rosmsg python-rostopic python-roswtf ros-core ros-base

Secondly I get:

The following packages have unmet dependencies:
catkin : Depends: python-catkin-pkg but it is not going to be installed
E: Unable to correct problems, you have held broken packages.
debian@beaglebone:~$ sudo apt install python-catkin-pkg
Reading package lists... Done
Building dependency tree       
Reading state information... Done
python-catkin-pkg is already the newest version (0.4.8-100).

So catkin wants python-catkin-pkg , which is said to be installed already. :( What do I have to do?