mx31ads: Convert to generic board

Boards need to select CONFIG_SYS_GENERIC_BOARD in order to prevent removal
from the project.

Acked-by: Anatolij Gustschin <agust@denx.de>
Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>

include/configs/mx31ads.h

@@ -14,6 +14,8 @@
  /* High Level Configuration Options */
 #define CONFIG_MX31		1		/* This is a mx31 */
 
+#define CONFIG_SYS_GENERIC_BOARD
+
 #define CONFIG_DISPLAY_CPUINFO
 #define CONFIG_DISPLAY_BOARDINFO