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/T4240RDB_SDCARD_defconfig b/configs/T4240RDB_SDCARD_defconfig
index 054131e..539d0e9 100644
--- a/configs/T4240RDB_SDCARD_defconfig
+++ b/configs/T4240RDB_SDCARD_defconfig
@@ -3,6 +3,7 @@
 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_MPC85xx=y
 CONFIG_TARGET_T4240RDB=y