Use `filter` instead of `findstring`

See #672
Eliminate findstring in more places
https://github.com/Kijewski/RIOT/commit/102dc453828012985f82915ad826be069f444e61#commitcomment-5378590
Fix cc110x vs cc110x_ng problems
Merge pull request #701 from Kijewski/issue-672

Use `filter` instead of `findstring`