New build info feature. (per @Analogreality request)

- New build info feature. Hidden command ‘$I’ will print the build info
for your Grbl firmware. Users may also write an identifying message
within it via ‘$I=‘ with up to 32 characters. (no more, or else it will
break).

- Adjusted the max number of startup lines to 3. Majority of people
will only need one.

- Fixed a compile error with spindle_control.c. A rogue #endif was
causing problems.

spindle_control.c

@@ -92,5 +92,3 @@ void spindle_run(int8_t direction, uint16_t rpm)
     current_rpm = rpm;
   }
 }
-
-#endif