Stm32f3 new try. Now based on upstream version from yesterday.

Hi guys. Sorry I was away for a while.

I did the following:

- Since the git commits were a mess because our students were working without git for a while, and then a big commit went to you. It was ugly, and now that libopencm3 upstream changed considerably, I wasn't feeling confident to do a rebase and try to fix the "problems", then I took a whole day to start with the most recent libopencm3 commit from you, and started to put slowly as grained as possible commits to include f3 support. I hope you can now take a look more easily and less painfully at it. 
- The commits go from: just put Makefiles and minimum files to start f3 support, to later commits with more functionally. All commits shouldn't brake any other fx functionality, so if you find some of this commits can be applied right away, please go ahead. I'll really love to see this in upstream/master. 
- Most of your suggestions from last time were taken into account to make all this commits, I think the only ones that were left are the ones related to changing the name of things from f24 to f234, and that some of this and some commons had to much "specialized" code for f3. This time I tried to make it as much reusuable code as possible, so it must look better, but I may see that it may be necessary to organize the code in some other ways. I would like to talk to you per chat or some other way to solve this (if necessary).
- The examples are now also in another repository as upstream is doing and soon I will make another pull request.
- Thank you for your patience! Some of the code has been made by undergrad students whom are gaining programming experience at the moment and you may see that in the code. The commits of this pull request, where made by me, and I tried to check, fix and embelish the code as much as I could (disclaimer: now I'm a bit sleepy :) ).