Only build kernels for enabled TARGET_ARCHes in make universe/tinderbox.

Previously, all of the kernels for a given TARGET were built if that
target was enabled. This was implemented by having each kernel built
via a universe_kernconf_<KERNEL> target that was depended on by a
universe_kernconfs target. However, this meant that if one did a
build with a limited set of TARGET_ARCH values for a given TARGET,
kernels could be built for which we hadn't built a world or toolchain.
For example, 'make TARGETS=mips TARGET_ARCHES_mips=mips64' would build
mips32 kernels.

Fix this by adding an extra layer of indirection in the kernel make
targets. universe_kernconf_<KERNEL> is now a dependency of a new
universe_kernconfs_<TARGET_ARCH>. universe_kernconfs in turn depends
on a list of universe_kernconfs_<target_arch> values, but only the
values enabled in TARGET_ARCHES_<TARGET>.

Reviewed by: imp
MFC after: 1 month
Differential Revision: https://reviews.freebsd.org/D23031