packagegroup.bbclass: set LICENSE and LIC_FILES_CHKSUM

packagegroup.bbclass: set LICENSE and LIC_FILES_CHKSUM

* Use "??=" in bitbake.conf to set LICENSE, so that it can overrided by
  packagegroup.bbclass and recipes.

* Use "?=" to set LICENSE and LIC_FILES_CHKSUM to MIT by default
  in packagegroup.bbclass, this won't impact any packagegroup recipes
  which use non-MIT license, since they can be overrided by the recipe.

Signed-off-by: Robert Yang <liezhi.yang@windriver.com>