Homing search sequence now compile-time option. New defaults.h file. Tidying up.

- The homing sequence is now a compile-time option, where a user can
choose which axes(s) move in sequence during the search phase. Up to 3
sequences. Works with the locating phase and the pull-off maneuver.

- New defaults.h file to store user generated default settings for
different machines. Mainly to be used as a central repo, but each set
may be select to be compiled in as a config.h define.

nuts_bolts.h

@@ -26,6 +26,7 @@
 #include <stdint.h>
 #include <stdbool.h>
 #include "config.h"
+#include "defaults.h"
 
 #define false 0
 #define true 1