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/BSC9131RDB_NAND_defconfig b/configs/BSC9131RDB_NAND_defconfig
index 1c71340..ad00622 100644
--- a/configs/BSC9131RDB_NAND_defconfig
+++ b/configs/BSC9131RDB_NAND_defconfig
@@ -1,4 +1,6 @@
 CONFIG_PPC=y
+CONFIG_SPL_NAND_SUPPORT=y
+CONFIG_SPL_SERIAL_SUPPORT=y
 CONFIG_MPC85xx=y
 CONFIG_TARGET_BSC9131RDB=y
 CONFIG_FIT=y
@@ -8,7 +10,6 @@
 CONFIG_SYS_EXTRA_OPTIONS="BSC9131RDB,NAND"
 CONFIG_BOOTDELAY=10
 CONFIG_SPL=y
-CONFIG_SPL_NAND_SUPPORT=y
 CONFIG_HUSH_PARSER=y
 # CONFIG_CMD_IMLS is not set
 # CONFIG_CMD_FLASH is not set