gen_defines.py: rename gen_defines.py to gen_legacy_defines.py

This rename is mostly to easy git managment and review so any changes or
the addition of the new gen_defines.py doesn't look like a diff against
the old code if you look at just that commit.

We keep changes to a minimum to just keep things building with the
renamed gen_legacy_defines.py.

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>