License COS under GPLv2

[ryuukumar]

Adds the GPLv2 license and marks all kernel source files under it.

• Why GPLv2? Mostly following Linux's lead here on kernel licenses. It's flexible enough to help the OS be used if it ever needs to be, but also copyleft (personal preference).
• Why not BSD/MIT? I want COS to remain open in whichever forms it takes))
• Why license it at all? By default, code is all rights reserved, and legally nobody can use it even if they wanted to. With a license there is some scope for other people to legally do things with/to this code ;)