Display: add a primitive status display.

This isn't pretty at all, but it shows the principle.
Unfortunately it also exploits a bug in the I2C sending mechanism,
I2C sending hangs a few seconds after reset.

clock.c

@@ -13,6 +13,7 @@
 #include	"debug.h"
 #include	"heater.h"
 #include	"serial.h"
+#include "display.h"
 #ifdef	TEMP_INTERCOM
 	#include	"intercom.h"
 #endif
@@ -84,6 +85,10 @@ static void clock_250ms(void) {
 	}
 
 	ifclock(clock_flag_1s) {
+    #ifdef DISPLAY
+      display_clock();
+    #endif
+
 		if (DEBUG_POSITION && (debug_flags & DEBUG_POSITION)) {
 			// current position
 			update_current_position();

display.h

@@ -35,6 +35,7 @@
 void display_init(void);
 void display_clear(void);
 
+void display_clock(void);
 void display_set_cursor(uint8_t line, uint8_t column);
 void display_writechar(uint8_t data);
 

display_ssd1306.c

@@ -10,6 +10,8 @@
 
 #include "displaybus.h"
 #include "font.h"
+#include "sendf.h"
+#include "dda.h"
 
 
 static const uint8_t PROGMEM init_sequence[] = {
@@ -86,6 +88,9 @@ void display_clear(void) {
 
   \param column The horizontal cursor position to set, in pixels. First
                 column is zero.
+
+  Use this for debugging purposes, only. Regular display updates happen in
+  display_clock().
 */
 void display_set_cursor(uint8_t line, uint8_t column) {
 
@@ -98,6 +103,18 @@ void display_set_cursor(uint8_t line, uint8_t column) {
   displaybus_write(0x10 | ((column >> 4) & 0x0F), 1);
 }
 
+/**
+  Regular update of the display. Typically called once a second from clock.c.
+*/
+void display_clock(void) {
+
+  display_set_cursor(0, 2);
+  update_current_position();
+  sendf_P(display_writechar, PSTR("X:%lq Y:%lq Z:%lq  F:%lu  "),
+          current_position.axis[X], current_position.axis[Y],
+          current_position.axis[Z], current_position.F);
+}
+
 /**
   Prints a character at the current cursor position.