Patch for rtems_waf to support build configurations in libbsd

[Christian Mauderer]

Hello Chris,

I now managed to build multiple configurations for libbsd. But I needed
the per-bsp-classes from the rtems_waf/rtems.py init function so that I
can derive further classes of them. So I added a hook to the init
function that is executed at the end.

If you want to know where I needed that hook, you can take a look at the
latest version of my ticket branch. Note that I still have to merge a
lot of commits on this branch to only a few clean ones. Basically every
commit that starts with FIXME:
https://git.rtems.org/christianm/rtems-libbsd.git/tree/wscript?h=ticket-3351&id=5aca4e1c04cc596a1202ff880ec186ce205fd817#n154

I added some basic notes regarding how the variants work (or at least
how I understood it) in the other thread:
https://lists.rtems.org/pipermail/devel/2018-April/020843.html

Would you see a better / simpler solution to that problem? If not: Would
you accept that patch?

Best regards

Christian Mauderer