package: allow Python packages with Python3

Until now, Python external modules were only visible when Python 2.x
was selected. With this commit, we now source all the Config.in files
of Python external modules, as soon as one of the two Python
interpreters is enabled.

Since all Python external modules have a "depends on
BR2_PACKAGE_PYTHON" in their Config.in, this commit in practice does
not allow to enable any Python external module. However, thanks to
this, we can progressively and safely enable more and more Python
external modules to build with Python 3, by simply changing their
dependency to "depends on BR2_PACKAGE_PYTHON || BR2_PACKAGE_PYTHON3".

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