Rescue images definitely don't need alterator pulled in.

Let's ensure that make-initrd-luks gets to the base install
until installer is tweaked to enable in-flight installation
of options like this.

Adding luks to stage1 [make-initrd] features makes no sense
on the other hand (and it wasn't happening anyways due to
the lack of add_feature function call in config.mk as was
accidentally spotted).

And putting luks packages into an installer image lacking
the reference to alterator-luks isn't that sensible, let's
complain to logs at the very least (this isn't going to hit
the default output though).

Installable live systems can have LUKS as well
and it's welcome addition to rescue images too.

It's a separate installation step to set the LUKS password;
see also #28200 for (terse) discussion and instructions on
getting this actually working for a distro.

That FIXME has been FIXED already.

Based on timonbl4@'s advice; might need some bleeding edge
packages right now, feel free to ask (everything should be
in place by p7/branch though).