main-afb-daemon: Export variables to connect to binder

Two new variables are exported that will allow
to connect to the binder:

 - AFB_PORT: the HTTP port that the binder listen
 - AFB_TOKEN: the initial token that the binder expects

These variables are defined only if the HTTP server
is started. They are available during initialisation
of bindings, even if it is before the real start of the
HTTP server.

Change-Id: I1b7c8572e3d77b7036a32af47b2a2f1d95803a41
Signed-off-by: José Bollo <jose.bollo@iot.bzh>