gf-prepare-kernel.sh: Fix device tree append

The default device tree variable sets the filename in the DEVICETREE_APPEND_FILE variable. However, after a successful overlay creation the variable also included the ${MOUNTPOINT}.

Fix this by removin the ${MOUNTPOINT} from the variable and add it to the place where the variable is being used. This re-enables platforms where the devcie tree overlay fails.

added 26 commits

• 72ac1d23...081e5f33 - 26 commits from branch dunfell

Compare with previous version

By Jonas Höppner on 2020-12-11T08:54:06 (imported from GitLab)

Already include as 61392bac

By Jonas Höppner on 2020-12-11T08:59:40 (imported from GitLab)