python: fix invalid library paths leaking into the build, and other improvements

This commit improves the cross-compilation patches we have on top of
Python, to fix the problem of host library paths leaking into the
build of target modules, as seen at:

 http://autobuild.buildroot.org/results/fcc/fccd7e08cd9d4713eb4208097dd48c5ab25749bc/build-end.log
 http://autobuild.buildroot.org/results/0bd/0bda780bf4b759b12edec26ac20b88cde617db4d/build-end.log



To do so, it ensures that the right python2.7/config/Makefile is used
when building target modules, and adjusts at runtime the paths read
from this Makefile if we are cross-compiling.

In addition, it installs the pgen program into the host directory, and
points the target python build to use python and pgen from $(HOST_DIR)
instead of from the host python source directory, which looks cleaner.

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