New port: Anbernic RG Nano

A bit of context, this device is a clone of the FunKey-S with a different form factor, hardware is mostly identical, the relevant difference is it has audio out (via usb-c, adapter to 3.5mm is included), this is the reason why the FunKey-SDK is needed for bulding.

This port is based on the old SDL 1.2 code because the device doesn't have SDL2 support. Alongside what was supported in the SDL 1.2 builds this port supports battery level, charging status and backlight control.

Change-Id: I7fcb85be62748644b667c0efebabf59d6e9c5ade

firmware/target/hosted/sdl/lcd-sdl.h

@@ -25,6 +25,10 @@
 #include "lcd.h"
 #include "SDL.h"
 
+#if SDL_MAJOR_VERSION == 1
+extern SDL_Surface *gui_surface;
+#endif
+
 void sdl_update_rect(SDL_Surface *surface, int x_start, int y_start, int width,
                      int height, int max_x, int max_y,
                      unsigned long (*getpixel)(int, int));