python: rework python symlinks installation

This change adds a patch to python disabling the installation of the
python and python-config symlinks.

This allows Buildroot to control these symlinks' installation:

* the python symlink should be unconditionally installed in the target
  tree, and the python-config symlink in the staging tree, since it is
  only built and installed in the target tree if the user selected it;

* the python and python-config symlinks should only be installed in
  the host tree when python(2) is the selection of the user for the
  target.
  Otherwise, when python3 is selected for the target, the host-python
  may be required to built some packages. In such cases, the python
  symlink should points to python3 (so should the python-config
  symlink) to reflect the staging/target tree.

[Thomas: fix comments according to Yann's suggestions, and replaced
python(2) by python2, as suggested by Yann.]

Signed-off-by: Samuel Martin <s.martin49@gmail.com>
Cc: Gustavo Zacarias <gustavo@zacarias.com.ar>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>