differenciate accel branch
possibly saving some time in the step interrupt
various optimisations in StepTicker and SteppperMotor
some reordering for speed
move back from 64bit to 32bits
fixing merge conflict
Merge branch 'edge' into accel
Move mpu.h to the correct location.

This header shouldn't have been located in the main Smoothie src tree
but in the build specific portion of the tree.  I think it got placed
in the incorrect location during my major merge just before pushing the
new build system into the edge branch.
Restructure mbed SDK makefiles.

No longer deploy common headers to drop subdirectory from the device
specific makefiles.  This caused contention during some parallel build
runs.  Now the common headers are deployed to drop from the main
makefile in the mbed/src directory.  This should not only remove the
parallel build race condition but even improve the performance of
since the common header deployment can now occur in parallel with
the device specific builds.
should make it work in octoprint
TemperatureControl: simplify handling of presets, fix preset not working for set-and-wait gcode
Pwm: limit _max to PID_PWM_MAX - 1

Reported-by: Hagrid
trying something. hopefully that can be a temp fix to the accel problem
Merge branch 'edge' into accel
temporary fix to the decel bug
removing debug pins and counter in StepTicker
PlayLed should not work this way
removing more debug pins
Added comments to most files in libs/
adding comments to modules/