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/birdland_bav335b_defconfig b/configs/birdland_bav335b_defconfig
index 6808231..e603522 100644
--- a/configs/birdland_bav335b_defconfig
+++ b/configs/birdland_bav335b_defconfig
@@ -9,6 +9,7 @@
 CONFIG_SPL_LIBDISK_SUPPORT=y
 CONFIG_SPL_MMC_SUPPORT=y
 CONFIG_SPL_POWER_SUPPORT=y
+CONFIG_SPL_SERIAL_SUPPORT=y
 CONFIG_BAV_VERSION=2
 CONFIG_FIT=y
 CONFIG_SYS_EXTRA_OPTIONS="SERIAL1,CONS_INDEX=1"