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/ls1043aqds_sdcard_qspi_defconfig b/configs/ls1043aqds_sdcard_qspi_defconfig
index 3e4850a..3aace54 100644
--- a/configs/ls1043aqds_sdcard_qspi_defconfig
+++ b/configs/ls1043aqds_sdcard_qspi_defconfig
@@ -4,6 +4,7 @@
 CONFIG_SPL_LIBGENERIC_SUPPORT=y
 CONFIG_SPL_I2C_SUPPORT=y
 CONFIG_SPL_MMC_SUPPORT=y
+CONFIG_SPL_SERIAL_SUPPORT=y
 CONFIG_SPL_ENV_SUPPORT=y
 CONFIG_SPL_DRIVERS_MISC_SUPPORT=y
 CONFIG_DEFAULT_DEVICE_TREE="fsl-ls1043a-qds-duart"