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".