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/C29XPCIE_NAND_defconfig b/configs/C29XPCIE_NAND_defconfig
index 691bf31..a393e7e 100644
--- a/configs/C29XPCIE_NAND_defconfig
+++ b/configs/C29XPCIE_NAND_defconfig
@@ -1,4 +1,6 @@
 CONFIG_PPC=y
+CONFIG_SPL_NAND_SUPPORT=y
+CONFIG_SPL_SERIAL_SUPPORT=y
 CONFIG_MPC85xx=y
 CONFIG_TARGET_C29XPCIE=y
 CONFIG_FIT=y
@@ -10,12 +12,12 @@
 CONFIG_SPL=y
 CONFIG_TPL=y
 CONFIG_SPL_DRIVERS_MISC_SUPPORT=y
-CONFIG_SPL_NAND_SUPPORT=y
 CONFIG_TPL_ENV_SUPPORT=y
 CONFIG_TPL_I2C_SUPPORT=y
 CONFIG_TPL_LIBCOMMON_SUPPORT=y
 CONFIG_TPL_LIBGENERIC_SUPPORT=y
 CONFIG_TPL_MPC8XXX_INIT_DDR_SUPPORT=y
+CONFIG_TPL_SERIAL_SUPPORT=y
 CONFIG_HUSH_PARSER=y
 CONFIG_CMD_SF=y
 CONFIG_CMD_I2C=y