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/cpu/armv7/omap3/Kconfig b/arch/arm/cpu/armv7/omap3/Kconfig
index 5cb6b81..7d884a2 100644
--- a/arch/arm/cpu/armv7/omap3/Kconfig
+++ b/arch/arm/cpu/armv7/omap3/Kconfig
@@ -30,6 +30,9 @@
 config SPL_POWER_SUPPORT
 	default y
 
+config SPL_SERIAL_SUPPORT
+	default y
+
 choice
 	prompt "OMAP3 board select"
 	optional