[OXT-23] Use common license directory for OXT recipes.

OXT has historically referenced the license in the TOPDIR of the
xenclient-oe repo. As we make this repo a meta layer TOPDIR ends up
pointing elsewhere. As such it's much easier to use the GPLv2 and MIT
license files from the common license dir provided by OE core.
Merge pull request #32 from flihp/master

[OXT-23] Use common license directory for OXT recipes.