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/x600_defconfig b/configs/x600_defconfig
index b953524..94b837b 100644
--- a/configs/x600_defconfig
+++ b/configs/x600_defconfig
@@ -2,6 +2,7 @@
 CONFIG_TARGET_X600=y
 CONFIG_SPL_LIBCOMMON_SUPPORT=y
 CONFIG_SPL_LIBGENERIC_SUPPORT=y
+CONFIG_SPL_SERIAL_SUPPORT=y
 CONFIG_BOOTDELAY=3
 CONFIG_SPL=y
 CONFIG_SPL_NOR_SUPPORT=y