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/sama5d4ek_spiflash_defconfig b/configs/sama5d4ek_spiflash_defconfig
index 6819cf2..65f59fa 100644
--- a/configs/sama5d4ek_spiflash_defconfig
+++ b/configs/sama5d4ek_spiflash_defconfig
@@ -4,6 +4,7 @@
 CONFIG_SPL_GPIO_SUPPORT=y
 CONFIG_SPL_LIBCOMMON_SUPPORT=y
 CONFIG_SPL_LIBGENERIC_SUPPORT=y
+CONFIG_SPL_SERIAL_SUPPORT=y
 CONFIG_FIT=y
 CONFIG_SYS_EXTRA_OPTIONS="SAMA5D4,SYS_USE_SERIALFLASH"
 CONFIG_BOOTDELAY=3