Use MACHINE_UUID in M115

Addressing #2879. The M115 report should display `MACHINE_UUID` but it
always displays `UUID`.

Marlin/language.h

@@ -83,8 +83,8 @@
   #error BUILD_VERSION Information must be specified
 #endif
 
-#ifndef UUID
-  #define UUID "00000000-0000-0000-0000-000000000000"
+#ifndef MACHINE_UUID
+  #define MACHINE_UUID "00000000-0000-0000-0000-000000000000"
 #endif
 
 
@@ -124,7 +124,7 @@
 #define MSG_INVALID_EXTRUDER                "Invalid extruder"
 #define MSG_INVALID_SOLENOID                "Invalid solenoid"
 #define MSG_ERR_NO_THERMISTORS              "No thermistors - no temperature"
-#define MSG_M115_REPORT                     "FIRMWARE_NAME:Marlin " DETAILED_BUILD_VERSION " SOURCE_CODE_URL:" SOURCE_CODE_URL " PROTOCOL_VERSION:" PROTOCOL_VERSION " MACHINE_TYPE:" MACHINE_NAME " EXTRUDER_COUNT:" STRINGIFY(EXTRUDERS) " UUID:" UUID "\n"
+#define MSG_M115_REPORT                     "FIRMWARE_NAME:Marlin " DETAILED_BUILD_VERSION " SOURCE_CODE_URL:" SOURCE_CODE_URL " PROTOCOL_VERSION:" PROTOCOL_VERSION " MACHINE_TYPE:" MACHINE_NAME " EXTRUDER_COUNT:" STRINGIFY(EXTRUDERS) " UUID:" MACHINE_UUID "\n"
 #define MSG_COUNT_X                         " Count X: "
 #define MSG_COUNT_A                         " Count A: "
 #define MSG_ERR_KILLED                      "Printer halted. kill() called!"