test: Run bootstd tests only on sandbox

These make use of disk images which are not available on reak boards.
Add a new Kconfig to ensure these tests only run where they are valid.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/test/Kconfig b/test/Kconfig
index 310fb1b..e842c01 100644
--- a/test/Kconfig
+++ b/test/Kconfig
@@ -65,6 +65,11 @@
 
 endif
 
+config UT_BOOTSTD
+	bool "Unit tests for standard boot"
+	depends on UNIT_TEST && SANDBOX
+	default y
+
 config UT_COMPRESSION
 	bool "Unit test for compression"
 	depends on UNIT_TEST