mx51evk: Add generic board support

Let's enable CONFIG_SYS_GENERIC_BOARD in order to get rid of warnings related
to generic board not being supported.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>

include/configs/mx51evk.h

@@ -26,6 +26,7 @@
 #define CONFIG_SETUP_MEMORY_TAGS
 #define CONFIG_INITRD_TAG
 #define CONFIG_REVISION_TAG
+#define CONFIG_SYS_GENERIC_BOARD
 
 #define CONFIG_OF_LIBFDT