meta-yadro: 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-yadro rev: a5126fbe78b18fbe1e52356fb7500cf528e74a65)

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: I9f89933f2d3892323400e7acad213a2a1e0c09eb
Signed-off-by: Andrew Geissler <geissonator@yahoo.com>