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/Makefile b/test/Makefile
index 8e1fed2..6b8a150 100644
--- a/test/Makefile
+++ b/test/Makefile
@@ -23,7 +23,7 @@
 obj-y += ut.o
 
 ifeq ($(CONFIG_SPL_BUILD),)
-obj-$(CONFIG_UNIT_TEST) += boot/
+obj-$(CONFIG_$(SPL_)UT_BOOTSTD) += boot/
 obj-$(CONFIG_UNIT_TEST) += common/
 obj-y += log/
 obj-$(CONFIG_$(SPL_)UT_UNICODE) += unicode_ut.o