libselinux: allow compiling python wrapper module for target

libselinux: allow compiling python wrapper module for target

libselinux currently does not compile its python wrapper module for
the target. This is needed for audit2allow to function properly, and
therefore this patch adjusts libselinux.mk to install the python
wrapper module is python or python3 are enabled.

Signed-off-by: Adam Duskett <Adamduskett@outlook.com>
Reviewed-by: Matt Weber <matthew.weber@rockwellcollins.com>
[Thomas:
 - Remove useless empty lines, as noted by Matt Weber
 - Move code related to python bindings before builds/install
   commands, since those commands will use variables defined by the
   python bindings logic.
 - Instead of enabling the python bindings when
   BR2_PACKAGE_POLICYCOREUTILS_AUDIT2ALLOW is set, enable the python
   bindings when python is available. We generally try to avoid
   looking at options of other packages to decide what to install.
 - Introduce LIBSELINUX_MAKE_TARGETS and
   LIBSELINUX_MAKE_INSTALL_TARGETS variable, in order to avoid
   duplicate the make/make install commands.
 - As suggested by Matt Weber, remove LIBSELINUX_PYTHONLIBDIR
   definitions, and don't pass PYLIBVER and PYTHONLIBDIR in MAKE_OPTS.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>