Fix the whole python building process to do proper crosscompiling for all python

Fix the whole python building process to do proper crosscompiling for all python
modules in openwrt. Thanks to ryd for helping to make this possible.
(Only numpy is completly broken now because it has dependencies which
where pervioulsy taken from the build system)

git-svn-id: svn://svn.openwrt.org/openwrt/packages@12821 3c298f89-4303-0410-b956-a3cf2f4a3e73