python: fix host-python symlink installation when no python is selected

When no python interpreter is selected, all host-python symlink
installation were disabled.
This could lead to a non-existing $(HOST_DIR)/usr/bin/python program.

Fixes:
  http://autobuild.buildroot.net/results/6e7/6e74a343b4e8e113fd34d45c9279419e233afe8a/



Signed-off-by: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>