Fix make kernels to match original commit message

make kernels was originally documented (in commit r295099) as the same as make
universe -DJUST_BUILD_KERNELS. However, it used the UNIVERSE_TARGET=buildkernel
which is subtly different in that it also builds the toolchains and doesn't
build the LINT modules unless they happened to already exist in the tree. This
unbreaks POLA and just builds the kernels, including LINT, now rather than all
that other stuff as well. When the commit originally happened, the two just
differed by the LINT bug. r335711 introduced the universe dependency on the
toolchain that wasn't present before, which diverged the two further. This
restores the original intent of r295099.

Reviewed by: brooks, bdrewery, emaste (LGTM)
MFC After: 3 days
Differential Revision: https://reviews.freebsd.org/D24212