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/mx6ul_9x9_evk_defconfig b/configs/mx6ul_9x9_evk_defconfig
index 7c7eec8..3616fa2 100644
--- a/configs/mx6ul_9x9_evk_defconfig
+++ b/configs/mx6ul_9x9_evk_defconfig
@@ -8,6 +8,7 @@
 CONFIG_SPL_I2C_SUPPORT=y
 CONFIG_SPL_LIBDISK_SUPPORT=y
 CONFIG_SPL_MMC_SUPPORT=y
+CONFIG_SPL_SERIAL_SUPPORT=y
 CONFIG_SYS_EXTRA_OPTIONS="IMX_CONFIG=arch/arm/imx-common/spl_sd.cfg"
 CONFIG_BOOTDELAY=3
 CONFIG_SPL=y