Aimed at live images at first but should cover installers as well.

This has been brewing for quite some time and while the proper
implementation is considerably more complex (and hard to do)
looks like there's demand for the particular important use case,
namely LiveCDs for Russian users, so this code has been shared
with a few people before merge.

These have been proofread somewhat to correspond
to the current state of affairs; a missing one
was added for fonts feature.

It might be spottable but not immediately obvious that a feature
lives entirely in features.in/FEATURE and every target it provides
is described in the corresponding config.mk; thanks dkr@ for asking.

It wasn't articulated clearly enough that features are strictly
incremental (while variables can be overridden).

Status updates and readability tweaks all over the place.

Somewhat more uniform style and a few additional bits.

Rather minor fixups for things changed in the meanwhile and not
yet (re)documented properly; and a change for memtest feature
to require syslinux feature (the code's been changed to fit
the updated description, actually, and the change is purely
formal as no syslinux alternative is being used/planned so far).

ToC:
- introduced doc/
- features.in/00example/
- READMEs and TODO for more
- code comments clarified

Some unrelated minor fixes bundled either.