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/pxm2_defconfig b/configs/pxm2_defconfig
index ed032c0..2c76d94 100644
--- a/configs/pxm2_defconfig
+++ b/configs/pxm2_defconfig
@@ -9,6 +9,7 @@
 CONFIG_SPL_LIBDISK_SUPPORT=y
 CONFIG_SPL_MMC_SUPPORT=y
 CONFIG_SPL_NAND_SUPPORT=y
+CONFIG_SPL_SERIAL_SUPPORT=y
 CONFIG_DEFAULT_DEVICE_TREE="am335x-pxm50"
 CONFIG_FIT=y
 CONFIG_BOOTDELAY=3