Feature oneshot125a

This pull request follows on from https://github.com/cleanflight/cleanflight/pull/172 as it has all the changes merged in from master since then.

It also has a bugfix for when looptime is longer than timer size (ie setting looptime to 8300 uS would give strange output previously, but now it does the right thing).

The attempt at getting CC3D PPM to work will probably fail, due to the way that the timer overflow is triggered.
No volatile is necessary here (and in rest of code).