meta-lenovo: 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-lenovo rev: 71e316c8833bd6cdb691b6b36ed581dc14b0b237)

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: I0277bc74158d4f4fcf9a9c9c9368b435c8494de6
Signed-off-by: Andrew Geissler <geissonator@yahoo.com>