package/dehydrated: new package

package/dehydrated: new package

dehydrated is an ACME client written in bash. It should be able to run
under zsh as well, but this hasn't been tested so it isn't enabled for
now.

Normally, we would want an init script to start dehydrated, and an
example configuration file. However, it is very difficult to do this
in a generic way in Buildroot:
- we normally don't have cron running;
- we have no standard location for webroot;
- we have no standard location for certificates;
- we have no standard way to restart/reload the webserver.
So instead, provide brief documentation of how to use dehydrated in the
help text.

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>