Allow USE_DISPLAY to be set externally

Author: jepler

runcpm-rp2040-dvi-usb/runcpm-pico/hardware/pico/feather_dvi.h

@@ -8,8 +8,9 @@
 #include "../../console.h"
 #include "../../arduino_hooks.h"
 
-#undef USE_DISPLAY
+#ifndef USE_DISPLAY
 #define USE_DISPLAY (1)
+#endif
 
 #ifndef USE_MSC
 #define USE_MSC (0)