python3: add python -> python3 symlink for the host variant

The target python3 depends on host-python3, but most of the scripts
call "python", so we need to ensure that $(HOST_DIR)/usr/bin/python
exists. This patch achieves this by creating a python -> python3
symbolic link in $(HOST_DIR), just like we are already doing for the
target Python 3.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>