Add guards against empty recipes/nonexistent images.

This is just a proof of concept at this point, trying to get `make` to
succeed.

Makefile

@@ -35,23 +35,35 @@ $(foreach release,$(BUILD_RELEASES), \
     $(eval $(call dynamic_yaml_target,$(family),$(release)))))
 
 %.img.sha256: %.img
-	echo $@
-	sha256sum $< > $@
+	if [ -e $< ]; then \
+		echo $@; \
+		sha256sum $< > $@; \
+	fi
 
 %.img.xz.sha256: %.img.xz
-	echo $@
-	sha256sum $< > $@
+	if [ -e $< ]; then \
+		echo $@; \
+		sha256sum $< > $@; \
+	fi
 
 %.img.xz: %.img
-	xz -f -k -z -9 $<
+	if [ -e $< ]; then \
+		xz -f -k -z -9 $<; \
+	fi
 
 %.img.bmap: %.img
-	bmaptool create -o $@ $<
+	if [ -e $< ]; then \
+		bmaptool create -o $@ $<; \
+	fi
 
 %.img: %.yaml
-	touch $(@:.img=.log)
-	time nice $(as_root) vmdb2 --verbose --rootfs-tarball=$(subst .img,.tar.gz,$@) --output=$@ $(subst .img,.yaml,$@) --log $(subst .img,.log,$@)
-	chmod 0644 $@ $(@,.img=.log)
+	if [ ! -s $< ]; then \
+		echo "W: skipping empty recipe ($<)"; \
+	else \
+		touch $(@:.img=.log); \
+		time nice $(as_root) vmdb2 --verbose --rootfs-tarball=$(subst .img,.tar.gz,$@) --output=$@ $(subst .img,.yaml,$@) --log $(subst .img,.log,$@); \
+		chmod 0644 $@ $(@,.img=.log); \
+	fi
 
 _ck_root:
 	[ `whoami` = 'root' ] # Only root can summon vmdb2 ☹