mac80211: build each variant in its own dir

mac80211: build each variant in its own dir

Having different build directories is the default when the package
Makefile defines more than one variant.

Mac80211 overrides PKG_BUILD_DIR, not taking different variants in
consideration, which causes clobbering the directories when both
variants are built.

When compiled with AUTOREMOVE=y, the effect is that the package is
unnecessarily rebuilt when the package is compiled again.

Wihout AUTOREMOVE, the problem is worse: the second variant will not be
rebuilt, and you end up with the smallbuffers variant being a copy of
the regular one.

Signed-off-by: Eneas U de Queiroz <cotequeiroz@gmail.com>