atari5200: conio now uses just four colors altogether

See discussion in PR #870.

include/atari5200.h

@@ -77,6 +77,12 @@ extern void atr5200std_joy[];        /* referred to by joy_static_stddrv[] */
 #include <_antic.h>
 #define ANTIC (*(struct __antic*)0xD400)
 
+/* conio color defines */
+#define COLOR_WHITE 0x00
+#define COLOR_RED   0x01
+#define COLOR_GREEN 0x02
+#define COLOR_BLACK 0x03
+
 /* The following #define will cause the matching function calls in conio.h
 ** to be overlaid by macros with the same names, saving the function call
 ** overhead.