build-vm: purge tarball even with DEBUG=1

...and leave it for possible further investigation
only if debug level is 2 or higher (which is uncommon).

features.in/build-vm/lib/90-build-vm.mk

@@ -41,7 +41,7 @@ convert-image: prepare-image
 		qemu-img convert -O "$$VM_FORMAT" \
 			"$(VM_RAWDISK)" "$(IMAGE_OUTPATH)"; \
 		rm "$(VM_RAWDISK)"; \
-		if [ -z "$(DEBUG)" ]; then rm "$(VM_TARBALL)"; fi; \
+		if [ "0$(DEBUG)" -le 1 ]; then rm "$(VM_TARBALL)"; fi; \
 	fi
 
 run-image-scripts: GLOBAL_CLEANUP_PACKAGES := $(CLEANUP_PACKAGES)