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/board/sunxi/Kconfig b/board/sunxi/Kconfig
index 76ed46a..79f644a 100644
--- a/board/sunxi/Kconfig
+++ b/board/sunxi/Kconfig
@@ -18,6 +18,9 @@
 config SPL_POWER_SUPPORT
 	default y
 
+config SPL_SERIAL_SUPPORT
+	default y
+
 # Note only one of these may be selected at a time! But hidden choices are
 # not supported by Kconfig
 config SUNXI_GEN_SUN4I