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.

report.c

@@ -125,7 +125,7 @@ void report_feedback_message(uint8_t message_code)
 // Welcome message
 void report_init_message()
 {
-  printPgmString(PSTR("\r\nGrbl " GRBL_VERSION " ("GRBL_VERSION_BUILD ") ['$' for help]\r\n"));
+  printPgmString(PSTR("\r\nGrbl " GRBL_VERSION " ['$' for help]\r\n"));
 }
 
 // Grbl help message
@@ -292,6 +292,16 @@ void report_startup_line(uint8_t n, char *line)
   printPgmString(PSTR("\r\n"));
 }
 
+
+// Prints build info line
+void report_build_info(char *line)
+{
+  printPgmString(PSTR("[" GRBL_VERSION "." GRBL_VERSION_BUILD ":"));
+  printString(line);
+  printPgmString(PSTR("]\r\n"));
+}
+
+
  // Prints real-time data. This function grabs a real-time snapshot of the stepper subprogram 
  // and the actual location of the CNC machine. Users may change the following function to their
  // specific needs, but the desired real-time data report must be as short as possible. This is