Add dependency for mozilla iot to webapps and some Makefile polishing

My point of view is that we should cooperate with upstream to integrate our web apps to be usable in upstream. Anyway, if you add a dependency for web apps to the app itself, we will need to rebase patches for it. But for now, just add a dependency for node-mozilla-iot-gateway to mozilla-iot-gateway-webapp.

Partially fixes: https://gitlab.labs.nic.cz/turris/turris-build/commit/56dfda77950a05eef55548c66a92779fd508a676
Closes #435 (closed)