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> Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
