libertas: rename persistcfg.c -> mesh.c

mesh.c will be the file where we concentrate all mesh-related
code. This allows us to either add a KConfig entry for mesh and
makes matters easier for the cfg80211 transition.

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