converting tabs to spaces in core (#1439)

This PR converts tabs to white spaces.
The statement I used for the conversion:
'''find . -name "*.[ch]" -exec zsh -c 'expand -t 4 "$0" > /tmp/e && mv /tmp/e "$0"' {} \;'''
Afterwards, I had a quick overview of the converted files to prevent odd indentation.
Merge pull request #1513 from cgundogan/remove_tabs_core

converting tabs to spaces in core (#1439)
doc: fix comment indentation
doc: use lgplv2.1-short license header instead of lgpl-short-riot
doc: use lgplv2.1-short license header instead of lgplv2-short-v3
doc: use lgplv2.1-short license header instead of lgplv2-short-v2
doc: use lgplv2.1-short license header instead of lgplv2-short-v1