[OE-core] LICENSE for recipes with empty SRC_URI

[Andre McCurdy]

There doesn't seem to be much consistency in the way LICENSE is
handled for recipes which don't need to define anything in SRC_URI.

 - Packagegroup recipes have a default LICENSE in packagegroup.bbclass
and generally don't define their own value, for reasons explained in:

  http://git.openembedded.org/openembedded-core/commit/?id=c04ae17f439ffd5fd70d8564430a94582e2cf688

 - Image recipes have a default LICENSE in image.bbclass but generally
always explicitly define the value again. A precedent for doing that
seems to have been set by:

  http://git.openembedded.org/openembedded-core/commit/?id=fb617bed6ebbb17ca9a14ea5985302b03311ccb7

 - SDK recipes (anything inheriting populate_sdk) don't have a default
LICENSE and must explicitly define one in each recipe.

Given that LICENSE is meant to apply to the sources associated with a
recipe, not the recipe itself, shouldn't all these cases be following
the approach currently taken by packagegroup recipes (ie a default set
by the appropriate class and no LICENSE in the recipe)? Or is there a
fundamental difference between these cases which justifies the three
different approaches?