Add Board Revision for Wireless chip

Gumstix Overo COMs with board revision 0x4 use a different Wifi and
Bluetooth module: Wi2Wi's W2CBW0015.  No other code changes are required
in u-boot---just handling of this particular board revision.

Signed-off-by: Ash Charles <ashcharles@gmail.com>
diff --git a/board/overo/overo.h b/board/overo/overo.h
index 64604de..25c9f16 100644
--- a/board/overo/overo.h
+++ b/board/overo/overo.h
@@ -22,6 +22,7 @@
 #define REVISION_1	0x1
 #define REVISION_2	0x2
 #define REVISION_3	0x3
+#define REVISION_4	0x4
 
 /*
  * IEN  - Input Enable