linux-dummy: set LICENSE to GPLv2 to avoid license warning

linux-dummy: set LICENSE to GPLv2 to avoid license warning

This recipe doesn't actually produce any non-empty packages, but since
it's pretending to be a Linux kernel it might as well pretend to have
the same license, if for no other reason than to avoid producing a
warning since the previous value ("GPL") isn't acceptable.

(From OE-Core rev: 92cbf6962b82f9118bdc55883851adf2b5fcc930)

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>