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/da850evm_defconfig b/configs/da850evm_defconfig
index 68de1ce..7b4cef3 100644
--- a/configs/da850evm_defconfig
+++ b/configs/da850evm_defconfig
@@ -3,6 +3,7 @@
 CONFIG_TARGET_DA850EVM=y
 CONFIG_SPL_LIBCOMMON_SUPPORT=y
 CONFIG_SPL_LIBGENERIC_SUPPORT=y
+CONFIG_SPL_SERIAL_SUPPORT=y
 CONFIG_SYS_EXTRA_OPTIONS="MAC_ADDR_IN_SPIFLASH"
 CONFIG_BOOTDELAY=3
 CONFIG_VERSION_VARIABLE=y