Convert CONFIG_SPL_LIBDISK_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 6c8a684..abb8563 100644
--- a/arch/arm/cpu/armv7/omap3/Kconfig
+++ b/arch/arm/cpu/armv7/omap3/Kconfig
@@ -15,6 +15,9 @@
 config SPL_LIBCOMMON_SUPPORT
 	default y
 
+config SPL_LIBDISK_SUPPORT
+	default y
+
 choice
 	prompt "OMAP3 board select"
 	optional