Allow to remove libmicrohttpd (HTTP server)

This allows to remove the dependency to libmicrohttpd.
At the end, this will allow either to replace it with
an other HTTP library or to fully remove the HTTP server.

It also makes a tiny improvement of the conditionnal compilation.

Change-Id: I24106659af04453b6693f2cda0939a601391202f
Signed-off-by: Jose Bollo <jose.bollo@iot.bzh>