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/ipam390_defconfig b/configs/ipam390_defconfig
index 426c84a..545d558 100644
--- a/configs/ipam390_defconfig
+++ b/configs/ipam390_defconfig
@@ -5,6 +5,7 @@
 CONFIG_SPL_LIBCOMMON_SUPPORT=y
 CONFIG_SPL_LIBGENERIC_SUPPORT=y
 CONFIG_SPL_NAND_SUPPORT=y
+CONFIG_SPL_SERIAL_SUPPORT=y
 CONFIG_VERSION_VARIABLE=y
 CONFIG_SPL=y
 CONFIG_HUSH_PARSER=y