This was asked for by Leo-sp50 and torabora, and seems quite reasonable:
let's provide means to keep at least some distribution configurations
a bit apart, so that these can be considered more standalone in terms
of hard warranted functionality but at the same time enjoying the common
infrastructure.

Considering lib/distro.mk: it's now experimentally pulled apart so that
parallel development of different distro families can go on without
major merge hassles.  *Please* don't abuse with massive copy-paste.

And before you ask: this might get extended to allow for "private"
out-of-tree configurations being included since apparently there
are goals with no meaning outside of some very particular context...
but otherwise I'd like to encourage getting reusable bits in-tree.