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/T1042D4RDB_NAND_defconfig b/configs/T1042D4RDB_NAND_defconfig
index a60cdfc..0f2cb81 100644
--- a/configs/T1042D4RDB_NAND_defconfig
+++ b/configs/T1042D4RDB_NAND_defconfig
@@ -2,6 +2,8 @@
 CONFIG_SPL_LIBCOMMON_SUPPORT=y
 CONFIG_SPL_LIBGENERIC_SUPPORT=y
 CONFIG_SPL_I2C_SUPPORT=y
+CONFIG_SPL_NAND_SUPPORT=y
+CONFIG_SPL_SERIAL_SUPPORT=y
 CONFIG_SPL_ENV_SUPPORT=y
 CONFIG_MPC85xx=y
 CONFIG_TARGET_T104XRDB=y
@@ -14,7 +16,6 @@
 CONFIG_SPL=y
 CONFIG_SPL_DRIVERS_MISC_SUPPORT=y
 CONFIG_SPL_MPC8XXX_INIT_DDR_SUPPORT=y
-CONFIG_SPL_NAND_SUPPORT=y
 CONFIG_HUSH_PARSER=y
 CONFIG_CMD_GREPENV=y
 CONFIG_CMD_MMC=y