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/brppt1_mmc_defconfig b/configs/brppt1_mmc_defconfig
index b63c7a0..257ca7b 100644
--- a/configs/brppt1_mmc_defconfig
+++ b/configs/brppt1_mmc_defconfig
@@ -6,6 +6,7 @@
 CONFIG_SPL_I2C_SUPPORT=y
 CONFIG_SPL_MMC_SUPPORT=y
 CONFIG_SPL_POWER_SUPPORT=y
+CONFIG_SPL_SERIAL_SUPPORT=y
 CONFIG_OF_BOARD_SETUP=y
 CONFIG_SYS_EXTRA_OPTIONS="SERIAL1,CONS_INDEX=1,EMMC_BOOT"
 CONFIG_BOOTDELAY=-2