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/at91sam9x5ek_nandflash_defconfig b/configs/at91sam9x5ek_nandflash_defconfig
index 766d1af..b8766fd 100644
--- a/configs/at91sam9x5ek_nandflash_defconfig
+++ b/configs/at91sam9x5ek_nandflash_defconfig
@@ -4,10 +4,11 @@
 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_SYS_EXTRA_OPTIONS="AT91SAM9X5,SYS_USE_NANDFLASH"
 CONFIG_BOOTDELAY=3
 CONFIG_SPL=y
-CONFIG_SPL_NAND_SUPPORT=y
 CONFIG_HUSH_PARSER=y
 CONFIG_SYS_PROMPT="U-Boot> "
 CONFIG_CMD_BOOTZ=y