spl, common, serial: build SPL without serial support

This patch enables building SPL without
CONFIG_SPL_SERIAL_SUPPORT support.

Signed-off-by: Heiko Schocher <hs@denx.de>
[trini: Ensure we build arch/arm/imx-common on mx28]
Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/include/configs/mx31pdk.h b/include/configs/mx31pdk.h
index 7e709cd..72d28c9 100644
--- a/include/configs/mx31pdk.h
+++ b/include/configs/mx31pdk.h
@@ -35,6 +35,7 @@
 #define CONFIG_SPL_MAX_SIZE	2048
 #define CONFIG_SPL_NAND_SUPPORT
 #define CONFIG_SPL_LIBGENERIC_SUPPORT
+#define CONFIG_SPL_SERIAL_SUPPORT
 
 #define CONFIG_SPL_TEXT_BASE	0x87dc0000
 #define CONFIG_SYS_TEXT_BASE	0x87e00000