heads up: new header guard rules

Hey fellow RIOTers,

we've just merged [1], which will enforce certain rules about the C header guards. See [2] for more details.

In order to adapt your git tree,

    $ dist/tools/headerguards/check.sh

... will show which files might need a header guard fix. The checker outputs unified diff, which can be piped directly into "patch -p0".

Cheers, Kaspar

[1] https://github.com/RIOT-OS/RIOT/pull/7095 [2] https://github.com/RIOT-OS/RIOT/wiki/Coding-conventions#header-guards