treewide: fix GPLv2+ obsolete-license warnings

The LICENSE string 'GPLv2+' is considered obsolete by Yocto and raises a
warning.  Switch them too the accepted tag 'GPL-2.0-or-later'.

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: I88c639dbb45762e2e019a3503f1f1930b65c09d6