HOMEAXIS: make into a function

Replace the large macro HOMEAXIS with a function.  This avoids the
compiler generating three copies of largely identical code.  The
saving is 724 bytes of program memory.

We make use of XYZ_CONSTS_FROM_CONFIG to provide convenient
array-shaped access to MAX_LENGTH, HOME_RETRACT_MM and HOME_DIR.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>