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/MPC8313ERDB_NAND_66_defconfig b/configs/MPC8313ERDB_NAND_66_defconfig
index fc9b953..a97f634 100644
--- a/configs/MPC8313ERDB_NAND_66_defconfig
+++ b/configs/MPC8313ERDB_NAND_66_defconfig
@@ -1,4 +1,6 @@
 CONFIG_PPC=y
+CONFIG_SPL_NAND_SUPPORT=y
+CONFIG_SPL_SERIAL_SUPPORT=y
 CONFIG_MPC83xx=y
 CONFIG_TARGET_MPC8313ERDB=y
 CONFIG_OF_BOARD_SETUP=y
@@ -6,7 +8,6 @@
 CONFIG_SYS_EXTRA_OPTIONS="SYS_66MHZ,NAND"
 CONFIG_BOOTDELAY=6
 CONFIG_SPL=y
-CONFIG_SPL_NAND_SUPPORT=y
 CONFIG_HUSH_PARSER=y
 CONFIG_CMD_I2C=y
 CONFIG_CMD_GPIO=y