Add "About Printer" information menu.

Includes:
  *firmware version / branch / date.
  *extruder count
  *board information (name, serial details, power supply type)
  *thermistors (names, min/max temperatures)
  *printer statistics (PRINTCOUNTER details)

Thanks to @thinkyhead for contributions.

Marlin/pins_MEGATRONICS_3.h

@@ -28,6 +28,7 @@
   #error "Oops!  Make sure you have 'Arduino Mega' selected from the 'Tools -> Boards' menu."
 #endif
 
+#define BOARD_NAME         "Megatronics v3.0"
 #define LARGE_FLASH        true
 
 #if ENABLED(Z_PROBE_SLED)