Make bld_app.py archive only the relevant directory after build

The build script executes make install in the end. Search for the
'lib' directory and archive the parent directory of that. No need to
have unnecessary nested directories in the archives.

Change-Id: Ia60672ccdae79189bdfb60f3984ef1a2d38d6175
Reviewed-by: Antti Kokko <antti.kokko@digia.com>