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/mx28evk_nand_defconfig b/configs/mx28evk_nand_defconfig
index 92f0729..d4f14a8 100644
--- a/configs/mx28evk_nand_defconfig
+++ b/configs/mx28evk_nand_defconfig
@@ -3,6 +3,7 @@
 CONFIG_SPL_GPIO_SUPPORT=y
 CONFIG_SPL_LIBCOMMON_SUPPORT=y
 CONFIG_SPL_LIBGENERIC_SUPPORT=y
+CONFIG_SPL_SERIAL_SUPPORT=y
 CONFIG_SYS_EXTRA_OPTIONS="ENV_IS_IN_NAND"
 CONFIG_BOOTDELAY=1
 CONFIG_VERSION_VARIABLE=y