Add support for supermin 5

[jani-matti]

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Support for supermin 5 would be nice. Without it creating bcache volumes with blocks won't work in e.g. Debian sid

[devurandom]

See-Also: g2p/blocks#13

[jean]

I tried to compile g2p/supermin on Ubuntu 15.04, but had to give up. After tracking down a lot of dependencies, I foundered on two issues:

• maintboot tries to create a supermin appliance containing the (non-existent) python3-blocks package;
• supermin: error: /etc/init.d/rc is a config file which is listed in two packages (/tmp/supermin2ed8e7e70181a4f894a8522c98b74e3c.tmp/openrc_0.13.1-4_amd64.deb, sysv-rc)

openrc is not installed, and I'm not sure where it's coming from.