[OE-core] License manifest OR handling

[Aníbal Limón]

Hi everybody,

I'm working in one bug [1] related to License manifest creation and i 
have some concerns about what is
expected in License creation more specifically in the OR's handling.

I re-implemented license creation function from shell to python for use 
oe.license module and added support
for take into account INCOMPATIBLE_LICENSES.

For example: Currently if you have LICENSE set to (MPLv2 | GPLv2.1) & 
LGPLv3,
     - With INCOMPATIBLE_LICENSES not set the result is: MPLv2 LGPLv3.
     - With INCOMPATIBLE_LICENSES set to MPLv2 the result is: GPLv2.1 
LGPLv3.

This is that oe.license model eval's the OR expression and if you have 
several OR only one is chosen, this is
the expected behavior?, if yes i think is needed to add priorities for 
license choose, example [2].

Comments?

Best regards.

[1] https://bugzilla.yoctoproject.org/show_bug.cgi?id=6765
[2] https://gist.github.com/kergoth/1590028