getInitfsModules: don't fail if kernel modules dir does not exist

This directory doesn't exist all the time, e.g. if the kernel was built
without modules or no modules were installed for some reason. Assume the
kernel package knows what it is doing and just print a message that
might be helpful if the kernel package ends up not knowing what it is
doing.

main.go

@@ -444,7 +444,9 @@ func getInitfsModules(files misc.StringSet, devinfo deviceinfo.DeviceInfo, kerne
 
 	modDir := filepath.Join("/lib/modules", kernelVer)
 	if !exists(modDir) {
-		return errors.New("Kernel module directory not found: " + modDir)
+		// dir /lib/modules/<kernel> if kernel built without module support, so just print a message
+		log.Printf("-- kernel module directory not found: %q, not including modules", modDir)
+		return nil
 	}
 
 	// modules.* required by modprobe