Convert CONFIG_SPL_SERIAL_SUPPORT to Kconfig

Move this option to Kconfig and tidy up existing uses.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/configs/mx31pdk_defconfig b/configs/mx31pdk_defconfig
index 03f5faf..a31ad5b 100644
--- a/configs/mx31pdk_defconfig
+++ b/configs/mx31pdk_defconfig
@@ -1,9 +1,10 @@
 CONFIG_ARM=y
 CONFIG_TARGET_MX31PDK=y
 CONFIG_SPL_LIBGENERIC_SUPPORT=y
+CONFIG_SPL_NAND_SUPPORT=y
+CONFIG_SPL_SERIAL_SUPPORT=y
 CONFIG_BOOTDELAY=1
 CONFIG_SPL=y
-CONFIG_SPL_NAND_SUPPORT=y
 CONFIG_CMD_BOOTZ=y
 # CONFIG_CMD_IMLS is not set
 CONFIG_CMD_SPI=y