Make temperature waiting independent from the movement queue.

The plan is to remove this stuff from the movement queue.

We still accept additional G-code ... until a G0 or G1 appears.
This e.g. allows to do homing or read temperature reports while
waiting.

Keep messages exactly as they were before, perhaps some Host
applications try to parse this.

This needs 2 bytes RAM and 138 bytes binary size. Performance is
unchanged. Let's see how this compares to the size reduction when
we remove the temperature handling code from the movement queue.

  ATmega sizes               '168   '328(P)   '644(P)     '1280
  Program:  19646 bytes      138%       64%       31%       16%
     Data:   2177 bytes      213%      107%       54%       27%
   EEPROM:     32 bytes        4%        2%        2%        1%

  short-moves.gcode statistics:
  LED on occurences: 888.
  LED on time minimum: 280 clock cycles.
  LED on time maximum: 458 clock cycles.
  LED on time average: 284.653 clock cycles.

  smooth-curves.gcode statistics:
  LED on occurences: 23648.
  LED on time minimum: 272 clock cycles.
  LED on time maximum: 501 clock cycles.
  LED on time average: 307.275 clock cycles.

  triangle-odd.gcode statistics:
  LED on occurences: 1636.
  LED on time minimum: 272 clock cycles.
  LED on time maximum: 458 clock cycles.
  LED on time average: 297.625 clock cycles.

clock.c

@@ -87,12 +87,25 @@ static void clock_250ms(void) {
   temp_heater_tick();
 
 	ifclock(clock_flag_1s) {
+    static uint8_t wait_for_temp = 0;
+
     #ifdef DISPLAY
       display_clock();
     #endif
 
     temp_residency_tick();
 
+    if (temp_waiting()) {
+      serial_writestr_P(PSTR("Waiting for target temp\n"));
+      wait_for_temp = 1;
+    }
+    else {
+      if (wait_for_temp) {
+        serial_writestr_P(PSTR("Temp achieved\n"));
+        wait_for_temp = 0;
+      }
+    }
+
 		if (DEBUG_POSITION && (debug_flags & DEBUG_POSITION)) {
 			// current position
 			update_current_position();

gcode_process.c

@@ -108,6 +108,7 @@ void process_gcode_command() {
 				//?
 				//? In this case move rapidly to X = 12 mm.  In fact, the RepRap firmware uses exactly the same code for rapid as it uses for controlled moves (see G1 below), as - for the RepRap machine - this is just as efficient as not doing so.  (The distinction comes from some old machine tools that used to move faster if the axes were not driven in a straight line.  For them G0 allowed any movement in space to get to the destination as fast as possible.)
 				//?
+        temp_wait();
 				backup_f = next_target.target.F;
 				next_target.target.F = MAXIMUM_FEEDRATE_X * 2L;
 				enqueue(&next_target.target);
@@ -121,6 +122,7 @@ void process_gcode_command() {
 				//?
 				//? Go in a straight line from the current (X, Y) point to the point (90.6, 13.8), extruding material as the move happens from the current extruded length to a length of 22.4 mm.
 				//?
+        temp_wait();
 				enqueue(&next_target.target);
 				break;
 
@@ -445,9 +447,7 @@ void process_gcode_command() {
 				//? --- M101: extruder on ---
 				//?
 				//? Undocumented.
-				if (temp_achieved() == 0) {
-					enqueue(NULL);
-				}
+        temp_wait();
 				#ifdef DC_EXTRUDER
 					heater_set(DC_EXTRUDER, DC_EXTRUDER_PWM);
 				#endif
@@ -648,8 +648,7 @@ void process_gcode_command() {
 				//? Example: M116
 				//?
 				//? Wait for temperatures and other slowly-changing variables to arrive at their set values.
-
-				enqueue(NULL);
+        temp_set_wait();
 				break;
 
       case 119:

temp.c

@@ -12,6 +12,7 @@
 #include	<stdlib.h>
 #include	"arduino.h"
 #include "serial.h"
+#include "clock.h"
 #include	"debug.h"
 #ifndef	EXTRUDER
 	#include	"sersendf.h"
@@ -101,6 +102,12 @@ static struct {
 #define TEMP_READ_CONTINUOUS (EWMA_ALPHA < EWMA_SCALE)
 #define TEMP_NOT_READY       0xffff
 
+/**
+  Flag for wether we currently wait for achieving temperatures.
+*/
+static uint8_t wait_for_temp = 0;
+
+
 /// Set up temp sensors.
 void temp_init() {
 	temp_sensor_t i;
@@ -550,6 +557,33 @@ uint8_t	temp_achieved() {
 	return all_ok;
 }
 
+/**
+  Note that we're waiting for temperatures to achieve their target. Typically
+  set by M116.
+*/
+void temp_set_wait() {
+  wait_for_temp = 1;
+}
+
+/**
+  Wether we're currently waiting for achieving temperatures.
+*/
+uint8_t temp_waiting(void) {
+  if (temp_achieved()) {
+    wait_for_temp = 0;
+  }
+  return wait_for_temp;
+}
+
+/**
+  Wait until all temperatures are achieved.
+*/
+void temp_wait(void) {
+  while (wait_for_temp && ! temp_achieved()) {
+    clock();
+  }
+}
+
 /// specify a target temperature
 /// \param index sensor to set a target for
 /// \param temperature target temperature to aim for

temp.h

@@ -42,6 +42,10 @@ void temp_residency_tick(void);
 
 uint8_t	temp_achieved(void);
 
+void temp_set_wait(void);
+uint8_t temp_waiting(void);
+void temp_wait(void);
+
 void temp_set(temp_sensor_t index, uint16_t temperature);
 uint16_t temp_get(temp_sensor_t index);