meta-evb: update Yocto compat for gatesgarth

The next release of Yocto is soon and most of the upstream
layers have switched support strings for it.  Support layer
compat for gatesgarth (current) and dunfell (previous).

(From meta-evb rev: 4d356d1b1afe5b7050220a649b894b1309973495)

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: Ia4a5889b5cb26171b4578f7a29254c9ad0c527a0
Signed-off-by: Andrew Geissler <geissonator@yahoo.com>