host-python-pyrex: use the HOST_*_NEEDS_HOST_PYTHON infrastructure

python-pyrex does not support python3.

So, using the *_NEEDS_HOST_PYTHON infrastructure will make sure we won't try
building it for python3. Besides, it will automatically take care of adding the
right host-python package to the dependency list.

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