Additional build info in the $I printout.

- [new] Added total available planner buffer blocks (15 shown, but
there are 16. one is used by the ring buffer and to execute system
motions) and serial RX buffer bytes. This information is useful for
GUIs to setup and optimize their streaming protocols easily.

[doc] Updated the interface document to reflect the change.

grbl/grbl.h

@@ -23,7 +23,7 @@
 
 // Grbl versioning system
 #define GRBL_VERSION "1.1f"
-#define GRBL_VERSION_BUILD "20170129"
+#define GRBL_VERSION_BUILD "20170131"
 
 // Define standard libraries used by Grbl.
 #include <avr/io.h>

grbl/report.c

@@ -433,9 +433,13 @@ void report_build_info(char *line)
   #ifndef HOMING_INIT_LOCK
     serial_write('L');
   #endif
+
   // NOTE: Compiled values, like override increments/max/min values, may be added at some point later.
-  // These will likely have a comma delimiter to separate them.   
-    
+  serial_write(',');
+  print_uint8_base10(BLOCK_BUFFER_SIZE-1);
+  serial_write(',');
+  print_uint8_base10(RX_BUFFER_SIZE);
+
   report_util_feedback_line_feed();
 }