Fix check for Python modules for cross-compilation

Here are two patches to enhance (fix) the check of Python modules while
cross-compiling.

There is extensive explanations in the commit log of each patch, but it
basically boils down to the fact that we can't check for Python modules
while doing cross-compilation:
  - we can't execute the target Python (obviously)
  - the host Python may not have the required module(s)

We've stumbled into this issue while building ola with Buildroot:
    http://buildroot.org/

Regards,
Yann E. MORIN.