pyterm: Use only printf-style String Formatting

Needed for backwards compatibility to python 2.6. Refers to
https://github.com/RIOT-OS/RIOT/commit/46c38230ebb7cbc690ac77d1b649e29c536f1d0c#commitcomment-7363298
Merge pull request #1568 from authmillenon/pyterm-printf-style

pyterm: Use only printf-style String Formatting