httpserver: fix make rules for submodules

Current rules were causing submodules to be updated even if .git
directories existed.

Reported-by: Amnon Heiman <amnon@cloudius-systems.com>
Signed-off-by: Tomasz Grabiec <tgrabiec@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>