Use the builtin RTC and display it

Its roughly %15 fast, and initializes to zero, but the display
currently shows HH:MM:SS AM/PM.

display.h

@@ -25,6 +25,7 @@
 #include <stdint.h>
 #include <stddef.h>
 #include <assert.h>
+#include <stdbool.h>
 
 #include "spi.h"
 #include "font.h"
@@ -52,6 +53,10 @@ struct display
 {
     SPI_TypeDef *spi;
     struct display_buffer buffer;
+
+    bool is_dirty;
+    uint8_t dirty_line_min;
+    uint8_t dirty_line_max;
 };