python: improve isolation of build environment and error handling

This commit mainly fixes bug #7268 by improving the isolation of
the build environment by making sure some host environment header
paths do not leak into the target build. The investigation was done by
David <buildroot-2014@inbox.com>. This is done by not calling the
add_multiarch_paths() function of setup.py when we're cross-compiling,
a change made in the newly introduced
python-013-dont-add-multiarch-path.patch.

In addition to this, another patch is added to make sure the build is
aborted when one of the Python module fails to build. This is done in
python-014-abort-on-failed-modules.patch. Without this, the Python
setup.py script simply logs which module failed to build, but doesn't
abort, so it's hard to notice when there is a problem.

[Peter: slightly reword commit message]
Cc: David <buildroot-2014@inbox.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/python/python-014-abort-on-failed-modules.patch

@@ -0,0 +1,21 @@
+Abort on failed module build
+
+When building a Python module fails, the setup.py script currently
+doesn't exit with an error, and simply continues. This is not a really
+nice behavior, so this patch changes setup.py to abort with an error,
+so that the build issue is clearly noticeable.
+
+Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
+
+Index: b/setup.py
+===================================================================
+--- a/setup.py
++++ b/setup.py
+@@ -281,6 +281,7 @@
+             print "Failed to build these modules:"
+             print_three_column(failed)
+             print
++            sys.exit(1)
+ 
+     def build_extension(self, ext):
+