trivial: If no devices support updates, show messaging (Closes: #1295)
trivial: If no devices support updates, show messaging (Closes: #1295)

Normally the flag 'supported' is used to indicate that devices can
update from a remote such as LVFS.

Running `#fwupdmgr get-updates` in this situation will show a message
for each device that is already up to date as well as messages for
devices that have updates available.

If no devices contain the supported flag, `# fwupdmgr get-updates`
will show no output, which is confusing to some people.

Show "No updatable devices" instead for those situations.

fwupd/fwupd,hughsie/fwupd,fwupd/fwupd,hughsie/fwupd,fwupd/fwupd,hughsie/fwupd,fwupd/fwupd,hughsie/fwupd