meta-portwell: 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-portwell rev: 6cfd36e2d61f2bde6adf84a09b3e0b3d4d2fb40b)

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: I9efd7ad33a43a9472d3475bd4ffb08d49d7d4be7
Signed-off-by: Andrew Geissler <geissonator@yahoo.com>