Change cancel_heatup to wait_for_heatup

Marlin/Marlin_main.cpp

 // Relative Mode. Enable with G91, disable with G90.
 static bool relative_mode = false;
 
-bool cancel_heatup = false;
+bool wait_for_heatup = true;
 
 const char errormagic[] PROGMEM = "Error:";
 const char echomagic[] PROGMEM = "echo:";
 
       // If command was e-stop process now
       if (strcmp(command, "M112") == 0) kill(PSTR(MSG_KILLED));
-      if (strcmp(command, "M108") == 0) cancel_heatup = true;
+      if (strcmp(command, "M108") == 0) wait_for_heatup = false;
 
       #if defined(NO_TIMEOUTS) && NO_TIMEOUTS > 0
         last_command_time = ms;
 /**
  * M108: Cancel heatup and wait for the hotend and bed, this G-code is asynchronously handled in the get_serial_commands() parser
  */
-inline void gcode_M108() {
-  cancel_heatup = true;
-}
+inline void gcode_M108() { wait_for_heatup = false; }
 
 /**
  * M109: Sxxx Wait for extruder(s) to reach temperature. Waits only when heating.
 
   float theTarget = -1.0, old_temp = 9999.0;
   bool wants_to_cool = false;
-  cancel_heatup = false;
+  wait_for_heatup = true;
   millis_t now, next_temp_ms = 0, next_cool_check_ms = 0;
 
   KEEPALIVE_STATE(NOT_BUSY);
       }
     }
 
-  } while (!cancel_heatup && TEMP_CONDITIONS);
+  } while (wait_for_heatup && TEMP_CONDITIONS);
 
   LCD_MESSAGEPGM(MSG_HEATING_COMPLETE);
   KEEPALIVE_STATE(IN_HANDLER);
 
     float theTarget = -1.0, old_temp = 9999.0;
     bool wants_to_cool = false;
-    cancel_heatup = false;
+    wait_for_heatup = true;
     millis_t now, next_temp_ms = 0, next_cool_check_ms = 0;
 
     KEEPALIVE_STATE(NOT_BUSY);
         }
       }
 
-    } while (!cancel_heatup && TEMP_BED_CONDITIONS);
+    } while (wait_for_heatup && TEMP_BED_CONDITIONS);
 
     LCD_MESSAGEPGM(MSG_BED_DONE);
     KEEPALIVE_STATE(IN_HANDLER);

Marlin/ultralcd.cpp

       stepper.quick_stop();
       print_job_timer.stop();
       thermalManager.autotempShutdown();
-      cancel_heatup = true;
+      wait_for_heatup = false;
       lcd_setstatus(MSG_PRINT_ABORTED, true);
       #if DISABLED(DELTA) && DISABLED(SCARA)
         set_current_position_from_planner();

Marlin/ultralcd.h

   extern int absPreheatHPBTemp;
   extern int absPreheatFanSpeed;
 
-  extern bool cancel_heatup;
+  extern bool wait_for_heatup;
 
   #if ENABLED(FILAMENT_LCD_DISPLAY)
     extern millis_t previous_lcd_status_ms;