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/work_92105_defconfig b/configs/work_92105_defconfig
index cd85a2d..d5e5b38 100644
--- a/configs/work_92105_defconfig
+++ b/configs/work_92105_defconfig
@@ -2,11 +2,12 @@
 CONFIG_TARGET_WORK_92105=y
 CONFIG_SPL_LIBCOMMON_SUPPORT=y
 CONFIG_SPL_LIBGENERIC_SUPPORT=y
+CONFIG_SPL_NAND_SUPPORT=y
+CONFIG_SPL_SERIAL_SUPPORT=y
 CONFIG_BOOTDELAY=3
 CONFIG_VERSION_VARIABLE=y
 CONFIG_SPL=y
 CONFIG_SPL_SYS_MALLOC_SIMPLE=y
-CONFIG_SPL_NAND_SUPPORT=y
 CONFIG_HUSH_PARSER=y
 # CONFIG_CMD_IMLS is not set
 # CONFIG_CMD_FLASH is not set