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/ti816x_evm_defconfig b/configs/ti816x_evm_defconfig
index d96c949..a214949 100644
--- a/configs/ti816x_evm_defconfig
+++ b/configs/ti816x_evm_defconfig
@@ -6,6 +6,7 @@
 CONFIG_SPL_FAT_SUPPORT=y
 CONFIG_SPL_LIBDISK_SUPPORT=y
 CONFIG_SPL_MMC_SUPPORT=y
+CONFIG_SPL_SERIAL_SUPPORT=y
 CONFIG_BOOTDELAY=3
 CONFIG_VERSION_VARIABLE=y
 CONFIG_SPL=y