NX: Add configurable callout to external logic to handle display update events

graphics/Kconfig

@@ -56,6 +56,30 @@ config NX_WRITEONLY
 		Automatically defined if NX_LCDDRIVER and LCD_NOGETRUN are
 		defined.
 
+config NX_UPDATE
+	bool "Display update hooks'
+	default n
+	---help---
+		Enable a callout to inform some external module that the display has
+		been updated.  This would be useful in a couple for cases.
+
+		- When a serial LCD is used, but a framebuffer is used to access the
+		  LCD.  In this case, the update callout can be used to refresh the
+		  affected region of the display.
+
+		- When VNC is enabled.  This is case, this callout is necessary to
+		  update the remote frame buffer to match the local framebuffer.
+
+		When this feature is enabled, some external logic must provide this
+		interface:
+
+		  void nx_notify_rectangle(FAR NX_PLANEINFOTYPE *pinfo,
+		                           FAR const struct nxgl_rect_s *rect);
+
+		That is the function that will handle the notification.  It
+		receives the rectangular region that was updated in the provided
+		plane.
+
 menu "Supported Pixel Depths"
 
 config NX_DISABLE_1BPP