tools: source packages reliably

[MilhouseVH]

As mentioned in #2141, package.mk should only ever be sourced with config/options.

I haven't updated tools/repo-tool as I'm not sure if it's still used/how it's used.

[Ray-future]

I'm using repo-tool but I never had any issues with it.

[MilhouseVH]

repo-tool should be updated then, as it could break at any time - it just needs someone to add a helper function in the global code section of a package and repo-tool will start complaining about command not found. I'll have a go at updating repo-tool this evening and update this PR.

[chewitt]

thanks, if repo-tool needs changes they can be added in a separate PR