Don't run fetch if rebuilding

Fetch was not working but the build was still fine, so fetch seems somewhat unnecessary. Fetch was fixed in another MR, so now fetch becomes yet even more time consuming if it is unnecessary. This MR addresses a couple of issues.

• Fetch of everything only happens the first time when you run make all.
• During make rebuild the fetch.tag is not deleted, so fetch does not run during rebuild and we rely on cook to update the source if needed.
• There was a bug that was causing an unnecessary rebuild when running make qemu the first time, caused by fstools.tag being out of date with respect to the cookbook directory after the cookbook/repo directory gets created. This has been fixed by touching fstools.tag after building the repo.

mk/repo.mk

@@ -20,8 +20,9 @@ else
 	cd cookbook && \
 	./repo.sh $(REPO_NONSTOP) "$${PACKAGES}"
 	mkdir -p $(BUILD)
-	# make sure fetch.tag is newer than the things repo modifies
-	touch $<
+	# make sure fstools.tag and fetch.tag are newer than the things repo modifies
+	touch $(FSTOOLS_TAG)
+	touch $(BUILD)/fetch.tag
 	touch $@
 endif