check is loader exist

Signed-off-by: Denis Zalevskiy <denis.zalevskiy@jolla.com>
[service] requires dbus.socket

While statefs itself has not any dependencies on d-bus, Some providers
need it and it is just simplier to add dependency on d-bus startup until
I'll found not so ugly way to solve the issue. Later I think there will
be a separate statefs service/instance for each provider

Signed-off-by: Denis Zalevskiy <denis.zalevskiy@jolla.com>