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/arch/arm/mach-rockchip/rk3288/Kconfig b/arch/arm/mach-rockchip/rk3288/Kconfig
index 473a357..94863a9 100644
--- a/arch/arm/mach-rockchip/rk3288/Kconfig
+++ b/arch/arm/mach-rockchip/rk3288/Kconfig
@@ -84,6 +84,9 @@
 config SPL_MMC_SUPPORT
 	default y
 
+config SPL_SERIAL_SUPPORT
+	default y
+
 source "board/chipspark/popmetal_rk3288/Kconfig"
 
 source "board/firefly/firefly-rk3288/Kconfig"