Adding Freescale Kinetis K20

This isn't complete, but just to make sure I'm structuring/formatting this appropriately.

I think the iomem.ld and the isr.rs could be generated by Ruby, there is lots of double up there. I'll have a look at how you have done the pinmap.rs.rb and see if it copies across well enough.

The SIM module is very big, so I have split that module into another folder. It controls the clock sources for everything though, so I need to get it working before I can even build app_blink.