Add ts-merge-images.sh script from git.linaro.org/configs repo

There is an issue with RockPi that is not copying a third partition. It's required to change ts-merge-images.sh to merge the third partition because the one from git.linaro.org is only doing it for two partitions. As it's required to change ts-merges-images.sh is better to host it in BP CI to continue maintaining here because on ledge is not required this kind of change. The ideal solution would be had only one script that works for most of the cases (ledge and BP CI/TRS), like using mergerfs. But, that will require more time and we need to unblock several tasks pending on this change.

ts-merge-images.sh is copied from https://git.linaro.org/ci/job/configs.git/tree/ledge/ts/scripts/ts-merge-images.sh

After merging it, it will be used by CI templates.

Signed-off-by: Javier Tia javier.tia@linaro.org