Fix the Python build when running Linux 3.*

When compiling Python on a host running Linux 3.0, the sys.platform
constant is set at "linux3". A lot of code (inside the interpreter
itself, its build system and third party apps and libraries) relies on
it to be linux2 on a Linux system.

This leads to the build of the target python package to break.

This behaviour has been reported on the Python bugtracker but is not
fixed yet.

http://bugs.python.org/issue12326



Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>