libertas: make mesh configurable

Mostly for the embedded people that know beforehand that they don't need
MESH at all and want to save some bytes, but also helpful for the upcoming
cfg80211 transition.

   text    data     bss     dec     hex filename
 114264    2308     140  116712   1c7e8 libertas.ko   with mesh
 105026    2000     140  107166   1a29e libertas.ko   without mesh
 --------------------------------------------------
  -9238    -308           -9546

Signed-off-by: Holger Schurig <holgerschurig@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>