python3: rework python symlinks installation

This patch reworks the way python3 and python3-config symlink are
installed.

Buildroot wants to control these symlinks' installation:

* the python3 symlink should be unconditionally installed in the target
  tree, and the python3-config symlink in the staging tree, since it is
  the only python package built and installed in the target tree if the
  user selected it;

* the python3 and python3-config symlinks should only be installed in
  the host tree when python3 is the selection of the user for the
  target.

[Thomas: fix comment 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>