bctoolbox: fix typos and logic error

To get the directory path from the library name you need to
use a single filename. Fix typo in variable name.

This fixes the ortp autobuild errors which uses the broken pkgconfig file
from bctoolbox.

Fixes:
  http://autobuild.buildroot.net/results/37d5625df4be11ccdc063871e9f6e13d5f59fb52
  http://autobuild.buildroot.net/results/1999c841fae41f860f00747a362327cb2857e687



Signed-off-by: Waldemar Brodkorb <wbx@openadk.org>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>