M206: save values in eeprom

Really, we should have a way to adjust the XYZ homing of a machine in
the eeprom.  So as the second stage of this, make the M206 home offset
parameters subject to the M500/M501/M502/M503 eeprom commands.

Bump the eeprom version to "V06".

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