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/mx28evk_auart_console_defconfig b/configs/mx28evk_auart_console_defconfig
index 94df592..e7bb119 100644
--- a/configs/mx28evk_auart_console_defconfig
+++ b/configs/mx28evk_auart_console_defconfig
@@ -3,6 +3,7 @@
 CONFIG_SPL_GPIO_SUPPORT=y
 CONFIG_SPL_LIBCOMMON_SUPPORT=y
 CONFIG_SPL_LIBGENERIC_SUPPORT=y
+CONFIG_SPL_SERIAL_SUPPORT=y
 CONFIG_SYS_EXTRA_OPTIONS="MXS_AUART,MXS_AUART_BASE=MXS_UARTAPP3_BASE,ENV_IS_IN_MMC"
 CONFIG_BOOTDELAY=1
 CONFIG_VERSION_VARIABLE=y