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/woodburn_sd_defconfig b/configs/woodburn_sd_defconfig
index 5412500..379d7fa 100644
--- a/configs/woodburn_sd_defconfig
+++ b/configs/woodburn_sd_defconfig
@@ -5,6 +5,7 @@
 CONFIG_SPL_LIBGENERIC_SUPPORT=y
 CONFIG_SPL_LIBDISK_SUPPORT=y
 CONFIG_SPL_MMC_SUPPORT=y
+CONFIG_SPL_SERIAL_SUPPORT=y
 CONFIG_SYS_EXTRA_OPTIONS="IMX_CONFIG=board/woodburn/imximage.cfg"
 CONFIG_BOOTDELAY=3
 CONFIG_SPL=y