Change license to GPLv2+

The Linux kernel is licensed under GPLv2. As GPLv3 and GPLv2 are incompatible in
some way, just license bbswitch as GPL 2.0 or (at your choice) any latter
version. Note: I wrote all code of bbswith.c, so I am entitled to change the
license and do not need permission from other authors (as there are not any).

Request made by Ludwig Nussel.