Housekeeping.

- Added some more notes to config.h.

- Added the ability to override some of the #defines around Grbl in
config.h, like planner buffer size, line buffer size, serial
send/receive buffers. Mainly to centralize the configurations to be
able to port to different microcontrollers later.

stepper.h

@@ -24,10 +24,6 @@
 
 #include <avr/io.h>
 
-// Some useful constants
-#define TICKS_PER_MICROSECOND (F_CPU/1000000)
-#define CYCLES_PER_ACCELERATION_TICK ((TICKS_PER_MICROSECOND*1000000)/ACCELERATION_TICKS_PER_SECOND)
-
 // Initialize and setup the stepper motor subsystem
 void st_init();