Convert CONFIG_SPL_EXT_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/omap4/Kconfig b/arch/arm/cpu/armv7/omap4/Kconfig
index 49adb8e..6a05625 100644
--- a/arch/arm/cpu/armv7/omap4/Kconfig
+++ b/arch/arm/cpu/armv7/omap4/Kconfig
@@ -1,5 +1,8 @@
 if OMAP44XX
 
+config SPL_EXT_SUPPORT
+	default y
+
 choice
 	prompt "OMAP4 board select"
 	optional