python3: cleanup python symlink installation

* Target install: Unconditionnaly install the python symlink
  Since python and python3 are exclusive for the target (ie. in Buildroot,
  it is not possible to have two python interperters in the target rootfs),
  there is no need to conditionnally create the python symlink in the target
  tree.

* Host install: Conditionnally install the python symlink (when python3 is
  also the python interpreter for the target).
  When python3 is not the python interpreter for target, then we do not want
  the host python symlink points to python3.

Signed-off-by: Samuel Martin <s.martin49@gmail.com>
Cc: Gustavo Zacarias <gustavo@zacarias.com.ar>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
python: handles the python symlink installation in the *.mk file

python: add patch removing the python symlink

We unconditionnally want the python symlink being created in the target
directory, because the python packages for the target are exclusive.

In the host tree, we want the python symlink reflects the choice for the
target interperter (ei. $(HOST_DIR)/usr/bin/python -> python3 if python3
is enabled, or $(HOST_DIR)/usr/bin/python -> python2 if python is enabled).

However, when python3 is enabled, we may also need python2 binary being
built and installed in the host tree to be able to run some build-system,
so build some other packages for the target.

So, this change add a patch removing the installation of the python symlink
by the python build-system, and defer this task at the Buildroot level, via
a post-install hooks.

python: fix symlink creation

Signed-off-by: Samuel Martin <s.martin49@gmail.com>
pkg-python: allow to defined a host-python deps different from the python in the target

pkg-python: invoke the right interpreter when _FORCE_HOST_PYTHON is set
scons: force host-python dependency to be python2

Scons build-system does not support python3 yet.
So, we need to force the host dependency to host-python to be able to
build scons-based packages, even if the python interpreter for the target
is python3.

scons: force python2 invokation
python-m2crypto: requires host-python2
crda: override python interperter

Using python2 to run kye2pub.py is mandatory because:
- the script is not python3 compliant (but could easily be)
- the script uses python-m2crypto, which is built against python2
  (cannot be built against python3 ?!?)