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.

settings.h

@@ -48,6 +48,7 @@
 #define EEPROM_ADDR_GLOBAL 1
 #define EEPROM_ADDR_PARAMETERS 512
 #define EEPROM_ADDR_STARTUP_BLOCK 768
+#define EEPROM_ADDR_BUILD_INFO 992
 
 // Define EEPROM address indexing for coordinate parameters
 #define N_COORDINATE_SYSTEM 6  // Number of supported work coordinate systems (from index 1)
@@ -93,6 +94,10 @@ void settings_store_startup_line(uint8_t n, char *line);
 // Reads an EEPROM startup line to the protocol line variable
 uint8_t settings_read_startup_line(uint8_t n, char *line);
 
+void settings_store_build_info(char *line);
+
+uint8_t settings_read_build_info(char *line);
+
 // Writes selected coordinate data to EEPROM
 void settings_write_coord_data(uint8_t coord_select, float *coord_data);