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 dunfell (previous).

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: I61c4ac459eac56d5f193b4b725e59f9d5ff36c2a
2 files changed