In this mode, both the metadata and firmware is stored on the local filesystem
and distributed using a distribution system like OSTree.
Fixes https://github.com/hughsie/fwupd/issues/162
If something changes the cache behind our back (e.g. deleting or updating the
file) we need to reload the list of remotes so that the age is correctly shown.
Note: we have to transfer the mtime (not the age) when creating the GVariant,
as we want calls to fwupd_remote_get_age() to update the value without getting
the remote from the daemon each time.
Add the concept of 'remotes' that can dropped into /etc and used as firmware
metadata sources. This may be desirable when firmware is only accessable with
a valid support contract or from behind a VPN.
