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/m53evk_defconfig b/configs/m53evk_defconfig
index 77f1774..9779ae4 100644
--- a/configs/m53evk_defconfig
+++ b/configs/m53evk_defconfig
@@ -3,12 +3,13 @@
 CONFIG_SPL_GPIO_SUPPORT=y
 CONFIG_SPL_LIBCOMMON_SUPPORT=y
 CONFIG_SPL_LIBGENERIC_SUPPORT=y
+CONFIG_SPL_NAND_SUPPORT=y
+CONFIG_SPL_SERIAL_SUPPORT=y
 CONFIG_FIT=y
 CONFIG_SYS_EXTRA_OPTIONS="IMX_CONFIG=board/denx/m53evk/imximage.cfg"
 CONFIG_BOOTDELAY=3
 CONFIG_VERSION_VARIABLE=y
 CONFIG_SPL=y
-CONFIG_SPL_NAND_SUPPORT=y
 CONFIG_HUSH_PARSER=y
 # CONFIG_CMD_IMLS is not set
 CONFIG_CMD_ASKENV=y