Everything is handled within mki-copy-efiboot currently
but it needs an image to process; extracting one from
bootloader branding seems less hassle than forcing it
into every flavour of branding.

This further refines the modular build by making
metadata being a clearly separated feature rather
than having to rely on runtime tests, and also by
moving the code which cares for kernel bits of base
installation (.base list) in a feature of its own.

There's more to it but let's get the ball rolling first.

The initial work covered live images but missed an installer bit
(thus notes and slideshow were missing in install2) while forgetting
to put branding packages into base list (thus kindly making these
available for *manual* installation sometime after, ouch).