meta-inventec: 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 dunsfell (previous).

(From meta-inventec rev: b1bb8dd37ec15e58fd2867724fb278649a6d3826)

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: Ia2c8e9334cc6beb473988ae040474f99ada741fd
Signed-off-by: Andrew Geissler <geissonator@yahoo.com>