Adding extra repositories for sfdk development

When doing manual builds or wanting to supply the SDK with external libraries. Proceed as follows:
1. setup: sfdk config target="SailfishOS-5.0.0.62-aarch64"
2. sfdk build-init
3. sfdk build-shell --maintain
4. su ar sailfishos-chum https://repo.sailfishos.org/obs/sailfishos:/chum/5.0_aarch64/
5. zypper install libupnp-devel libupnpp (for instance)
6. exit and go

Given the 'target' you will have used a root shell to add the chum repos and can now use libraries from chum for the local build in either the SDK or using sfdk make and co.