bootctl: make sure bootctl --image= works on image with /usr/ but without /

Let's make sure we can use the tool on ParticleOS images. They have no
root fs by default (until they are instantiated), but always have /usr/.
Hence add DISSECT_IMAGE_USR_NO_ROOT which has the desired effect.

src/bootctl/bootctl.c

@@ -718,6 +718,7 @@ static int run(int argc, char *argv[]) {
                                 arg_image,
                                 arg_image_policy,
                                 DISSECT_IMAGE_GENERIC_ROOT |
+                                DISSECT_IMAGE_USR_NO_ROOT |
                                 DISSECT_IMAGE_RELAX_VAR_CHECK |
                                 DISSECT_IMAGE_ALLOW_USERSPACE_VERITY,
                                 &mounted_dir,