allow switches to be updated automatically

[KanjiMonster]

To make it easier to users to stay up to date, allow switches to automatically update themselves at scheduled times.

For this to work, we need to have multiple things:

a. store the information about current releases in a machine readable file
b. write a program that reads that information to know if a new release is available
c. have a service that does this at configurable intervals, and update the switch if configured

This can probably implemented in multiple steps:

1. define a machine readable description of the image (filename, version, supported platforms ...) and publish it at a defined place
2. allow e.g. bisdn-linux-upgrade to download that, compare it with the installed version, and print if a new version is available
3. extend e.g. bisdn-linux-upgrade to also use it to trigger the installation
4. create a service that can be configured to to that at certain times

Optional things I can think of:

• allow configuring the versions to install/check for (e.g. '', '5.', '5.2.*' etc)
• pre-download the image once there is one available
• email? for any events (new version, install is scheduled, install is finished)

Reference: https://github.com/openwrt/asu