bootstd: Rework BLK dependency

The bootstd code itself does not have any dependency on BLK in order to
build. However, in order to minimize size growth of non-migrated
platforms, change this from being "default y" to "default y if BLK".
This will make it easier to begin migration of platforms which do not
have any BLK-class device but do want to use bootstd.

Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/boot/Kconfig b/boot/Kconfig
index 2456856..30eb5b3 100644
--- a/boot/Kconfig
+++ b/boot/Kconfig
@@ -427,8 +427,8 @@
 
 menuconfig BOOTSTD
 	bool "Standard boot"
-	default y
-	depends on DM && OF_CONTROL && BLK
+	default y if BLK
+	depends on DM && OF_CONTROL
 	help
 	  U-Boot supports a standard way of locating something to boot,
 	  typically an Operating System such as Linux, provided by a distro such