Support for other build architectures

Created by: anjos

In the context of a demonstrator here at Idiap, we have to compile Bob packages to work in a Raspberry Pi. After some tweaking on this package, it works fine. Here are the changes required to make this package more generic:

1. Remove all occurences of -mtune=generic and replace that with -mtune=native. Not all versions of gcc for different architectures support "-mtune=generic".
2. Inside bob.extension.utils, the find_library method needs to take into consideration a 32-bit ARM directory for the Raspberry PI as well, called arm-linux-gnueabihf.