libnice, farsight2: unblacklist as libnice's nice.pc issue fixed.

* Update do_compile_append where it was updating *.pc from ${S} and
  removing staging dir folder. Now we have seperate build folder
  where packages get build. We have to search pc in ${B} not in ${S}.
  After updating the tmp dir path is removed from nice.pc file.
* Remove PNBLACKLIST for libnice and farsight2 recipes.

Signed-off-by: Noor Ahsan <noor_ahsan@mentor.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>
Signed-off-by: Armin Kuster <akuster808@gmail.com>