Add SBHX configuration.scad
Tweak greg's extruder to use 4 of M3x45 screws.

Use M3 nuts/bolts for the idler springs.
Make the holes deeper to allow mounting with 45mm long M3 bolts.
Use captive nuts on Y belt holder

With a wood plate the holes can't be tapped, so reverse the direction of
the bolts and use captive nuts like the X carriage does.
Make Z top mounts taller for shorter frame.

We reduced frame to 300mm tall from 370ish, and compensate by making Z
top rod mounts taller.
Tweak endstop mounts for chosen switches, z-screw