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/apf27_defconfig b/configs/apf27_defconfig
index 3816f01..ffaab8a 100644
--- a/configs/apf27_defconfig
+++ b/configs/apf27_defconfig
@@ -1,9 +1,10 @@
 CONFIG_ARM=y
 CONFIG_TARGET_APF27=y
+CONFIG_SPL_NAND_SUPPORT=y
+CONFIG_SPL_SERIAL_SUPPORT=y
 CONFIG_BOOTDELAY=5
 CONFIG_VERSION_VARIABLE=y
 CONFIG_SPL=y
-CONFIG_SPL_NAND_SUPPORT=y
 CONFIG_HUSH_PARSER=y
 CONFIG_SYS_PROMPT="BIOS> "
 # CONFIG_CMD_IMLS is not set