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/sandbox_spl_defconfig b/configs/sandbox_spl_defconfig
index 9c2db90..82e481d 100644
--- a/configs/sandbox_spl_defconfig
+++ b/configs/sandbox_spl_defconfig
@@ -1,5 +1,6 @@
 CONFIG_SPL_LIBCOMMON_SUPPORT=y
 CONFIG_SPL_LIBGENERIC_SUPPORT=y
+CONFIG_SPL_SERIAL_SUPPORT=y
 CONFIG_SPL_ENV_SUPPORT=y
 CONFIG_SPL_DRIVERS_MISC_SUPPORT=y
 CONFIG_MMC=y