The various *8168 and friends among kernel modules
have finally been pushed into a designated target
so that RM doesn't have to care which particular
additional ethernet modules are available in this
particular branch and kernel.

Tweak distros as appropriate.

NB: *maybe* this is required by distro/.base either.

It wasn't spotted when introducing the distinct kernel feature;
will need fixup as we meet packaged ARM/PPC kernels.

un-def was a desperate measure when std-def of the day
was suddenly plain broken.

This reverts 0e8871e7.

3.5.2-std-def-alt2 brings boot problems which were absent
with 3.4.x-std-def and are absent with 3.5.x-un-def;
seems like it's better to stay with known good variant
at the moment instead of having to fall back to it.

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.