sandbox: Add a dummy booti command

Add basic sandbox support for 'booti' so we can start to boot the test
ARMbian image. This is helpful in checking that it is parsed correctly.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/cmd/booti.c b/cmd/booti.c
index 2db8f4a..41d40c9 100644
--- a/cmd/booti.c
+++ b/cmd/booti.c
@@ -75,7 +75,7 @@
 	unmap_sysmem((void *)ld);
 
 	ret = booti_setup(ld, &relocated_addr, &image_size, false);
-	if (ret != 0)
+	if (ret || IS_ENABLED(CONFIG_SANDBOX))
 		return 1;
 
 	/* Handle BOOTM_STATE_LOADOS */