Fix the kernel panic in hostap mode.
rvp->beacon_mbuf was NULL in run_update_beacon().

PR:	189405
Submitted by:	Gabor Simon <gabor.simon75 at gmail.com>
MFC after:	3 days
My previous commit exposed an issue as it fixed a different
issue. lib/atf isn't a prereq_lib, since it isn't required for other
libraries to build. Remove it. The old kludge of always building it
had effectively been retired. Since we don't want to build the
libraries with the tests when we're bootstrapping, invent
MK_TESTS_SUPPORT which normally defaults to the current MK_TESTS
value, except when explicitly defined. Make lib/atf depend on it being
yes. When building the libraries set MK_TESTS to no, and
MK_TESTS_SUPPORT to the current value of MK_TESTS so that later stages
of the build work correctly. This should fix (and does for me)
people's issues with parallel builds racing between lib/atf and
libexec/atf. Since lib/atf is built during the libraries phase, the
race disappears.