rockboy: fix wrong colors on some targets

This bug was found and located by a forum user, see
https://forums.rockbox.org/index.php/topic,54776.0.html

The old code used the FB_SCALARPACK macro which doesn't
respect swapped byte order.
Using FB_RGBPACK fixes it and furthermore makes the
fb.cc[] array superfluous.
I added a small optimization for 16 bit targets, since
the gameboy colors almost fit perfectly (5 bit per color).

Change-Id: If96b3943cb4ab902b880ff3d471add25e138fe8e

apps/plugins/rockboy/fb.h

@@ -10,12 +10,7 @@
 
 struct fb
 {
-#ifdef HAVE_LCD_COLOR
-    struct
-    {
-        int l, r;
-    } cc[3];
-#else
+#ifndef HAVE_LCD_COLOR
     int mode;
 #endif
     int enabled;