Add M220 B/R with PRUSA_MMU2 (#15739)

Marlin/src/gcode/config/M220.cpp

@@ -24,11 +24,23 @@
 #include "../../module/motion.h"
 
 /**
- * M220: Set speed percentage factor, aka "Feed Rate" (M220 S95)
+ * M220: Set speed percentage factor, aka "Feed Rate"
+ *
+ * Parameters
+ *   S<percent> : Set the feed rate percentage factor
+ *
+ * With PRUSA_MMU2...
+ *   B : Flag to back up the current factor
+ *   R : Flag to restore the last-saved factor
  */
 void GcodeSuite::M220() {
 
-  if (parser.seenval('S'))
+  #if ENABLED(PRUSA_MMU2)
-    feedrate_percentage = parser.value_int();
+    static int16_t backup_feedrate_percentage = 100;
+    if (parser.seen('B')) backup_feedrate_percentage = feedrate_percentage;
+    if (parser.seen('R')) feedrate_percentage = backup_feedrate_percentage;
+  #endif
+
+  if (parser.seenval('S')) feedrate_percentage = parser.value_int();
 
 }

Marlin/src/gcode/gcode.h

@@ -177,6 +177,7 @@
  * M217 - Set filament swap parameters: "M217 S<length> P<feedrate> R<feedrate>". (Requires SINGLENOZZLE)
  * M218 - Set/get a tool offset: "M218 T<index> X<offset> Y<offset>". (Requires 2 or more extruders)
  * M220 - Set Feedrate Percentage: "M220 S<percent>" (i.e., "FR" on the LCD)
+ *        Use "M220 B" to back up the Feedrate Percentage and "M220 R" to restore it. (Requires PRUSA_MMU2)
  * M221 - Set Flow Percentage: "M221 S<percent>"
  * M226 - Wait until a pin is in a given state: "M226 P<pin> S<state>"
  * M240 - Trigger a camera to take a photograph. (Requires PHOTO_GCODE)