Add support for arbitrary bitmaps for TRAB's VFD command;

allow to pass boot bitmap addresses in environment variables;
allow for zero boot delay

board/trab/vfd.c

@@ -346,9 +346,10 @@ void transfer_pic(int display, unsigned char *adr, int height, int width)
 				}
 			}
 		}
-		display++;
-		if (display > 3)
-			display = 0;
+		if (display > 0)
+			display--;
+		else
+			display = 3;
 	}
 }