Use variant kernel configs to simplify sys/*/conf

[cperciva]

Summary

We no longer need to provide kernel configuration files for e.g.
GENERIC-NODEBUG since those can be synthesized automatically by the
build system as GENERIC + std.nodebug.

Add std.* files for kasan, kcsan, kmsan, noinet, noip, noinet6, and
novimage.

Add new "UNIVERSE-EXTRAS" files to tell the universe-kernels target
which variant kernels to build in addition to "whatever is listed in
the sys/$arch/conf directory".

[cperciva]

Depends on D51602.

[cperciva]

I have no idea why Phabricator says I'm planning changes to this.

[imp]

The fact we are adding a dozen std.foo for a single option tells me this design is weak and we'll have lots of problems with it in the future.... I'm not officially objecting, but I'd be curios what ithers who have hacked on config thing.

[cperciva]

In D51616#1179424, @imp wrote:

The fact we are adding a dozen std.foo for a single option tells me this design is weak and we'll have lots of problems with it in the future.... I'm not officially objecting, but I'd be curios what ithers who have hacked on config thing.

That's a very good point. I'm going to rework this so that FOO-BAR means "include std.bar if that exists; otherwise add 'options bar'". Similarly for FOO-NOBAR. That will get rid of all of the new std.* files except for std.noip (which is nontrivial) and also allow for e.g. GENERIC-TSLOG without needing a new std.tslog file to be added.