buildroot

scanpypi: rework runtime dependency handling

In most cases Python's package dependencies found in setup.py are
runtime dependencies and hence don't need to be mentioned in *.mk
file.

Also add '# runtime' tag to select statements in Config.in.

__create_mk_requirements() itself is left for future uses (cffi backend
handling etc.).

Signed-off-by: Yegor Yefremov <yegorslists@googlemail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>