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/omapl138_lcdk_defconfig b/configs/omapl138_lcdk_defconfig
index 04cfc44..9da7e46 100644
--- a/configs/omapl138_lcdk_defconfig
+++ b/configs/omapl138_lcdk_defconfig
@@ -3,6 +3,7 @@
 CONFIG_TARGET_OMAPL138_LCDK=y
 CONFIG_SPL_LIBCOMMON_SUPPORT=y
 CONFIG_SPL_LIBGENERIC_SUPPORT=y
+CONFIG_SPL_SERIAL_SUPPORT=y
 CONFIG_BOOTDELAY=3
 CONFIG_VERSION_VARIABLE=y
 CONFIG_SPL=y