Allow importing custom build profiles

We use a class to store the build profiles. The same class could be used by users to create a custom build profiles. The user would create a 'user_build_profiles.py' in $USER/site_scons and this could be automatically imported and the original build profiles dict updated.

The user would be required to import the build profiles in there custom file, and populate a known dict USER_BUILD_PROFILES with the names and build profiles instances.