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.c

@@ -50,6 +50,12 @@ void settings_store_startup_line(uint8_t n, char *line)
   memcpy_to_eeprom_with_checksum(addr,(char*)line, LINE_BUFFER_SIZE);
 }
 
+// Method to store build info into EEPROM
+void settings_store_build_info(char *line)
+{
+  memcpy_to_eeprom_with_checksum(EEPROM_ADDR_BUILD_INFO,(char*)line, LINE_BUFFER_SIZE);
+}
+
 // Method to store coord data parameters into EEPROM
 void settings_write_coord_data(uint8_t coord_select, float *coord_data)
 {  
@@ -120,6 +126,19 @@ uint8_t settings_read_startup_line(uint8_t n, char *line)
   }
 }
 
+// Reads startup line from EEPROM. Updated pointed line string data.
+uint8_t settings_read_build_info(char *line)
+{
+  if (!(memcpy_from_eeprom_with_checksum((char*)line, EEPROM_ADDR_BUILD_INFO, LINE_BUFFER_SIZE))) {
+    // Reset line with default value
+    line[0] = 0;
+    settings_store_build_info(line);
+    return(false);
+  } else {
+    return(true);
+  }
+}
+
 // Read selected coordinate data from EEPROM. Updates pointed coord_data value.
 uint8_t settings_read_coord_data(uint8_t coord_select, float *coord_data)
 {