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/sama5d3xek_nandflash_defconfig b/configs/sama5d3xek_nandflash_defconfig
index fc01bd2..2ed8a33 100644
--- a/configs/sama5d3xek_nandflash_defconfig
+++ b/configs/sama5d3xek_nandflash_defconfig
@@ -4,11 +4,12 @@
 CONFIG_SPL_GPIO_SUPPORT=y
 CONFIG_SPL_LIBCOMMON_SUPPORT=y
 CONFIG_SPL_LIBGENERIC_SUPPORT=y
+CONFIG_SPL_NAND_SUPPORT=y
+CONFIG_SPL_SERIAL_SUPPORT=y
 CONFIG_FIT=y
 CONFIG_SYS_EXTRA_OPTIONS="SAMA5D3,SYS_USE_NANDFLASH"
 CONFIG_BOOTDELAY=3
 CONFIG_SPL=y
-CONFIG_SPL_NAND_SUPPORT=y
 CONFIG_HUSH_PARSER=y
 CONFIG_CMD_BOOTZ=y
 # CONFIG_CMD_IMI is not set