This configuration drives a standard 4-axis 3D printer using the PRU for
high speed step/dir pulse generation and a BeBoPr driver board.  The
BeBoPr configuration is for the original version of the BeBoPr board and a BeagleBoneWhite and
requires you disable HDMI.

The BeBoPr-Bridge configuration is for use with the new bridge board
that shuffles pins around so you can leave HDMI enabled.

There are currently no limit switches configured, so adjust the MIN_LIMIT
and MAX_LIMIT axis settings in the ini file if your printer has less than
200 mm of X/Y travel.

Current X and Y settings are appropriate for a typical "MendelMax" style
printer, with the following settings:

max vel   = 200 mm/s
max accel = 3000 mm/s/s
scale     = 80 steps/mm

Adjust as required for your machine.  Make scale negative to invert any
axis that is moving 'backwards'.