MFC r319859, r319888:

Add support for "make universe_kernels -DMAKE_GENERIC_KERNELS" to build
just the GENERIC kernels for each arch (including variations such as
GENERIC-NODEBUG, GENERIC64, etc).
This helps with quickly doing a test build for all[*] arches without
building dozens of variant kernels for the arches that have lots of
hardware/board/system variations.

• Not all arches have a generic kernel (but they probably should for

test-building purposes, even if it can't boot on any real hardware).

By popular demand: change MAKE_GENERIC_KERNELS to MAKE_LINT_KERNELS.
It appears that the same arches that lack GENERIC kernel configs also lack
LINT. But enough different arches get built to ensure a kernel change
should build everywhere (32 and 64 bit, clang and old gcc, little and big
endian).