[Toaster] Design - 8422 Setting DL_DIR and SSTATE_DIR in Toaster

[Barros Pena, Belen]

Design document attached to Bugzilla entry, as usual

https://bugzilla.yoctoproject.org/show_bug.cgi?id=8422

This also somehow affects

https://bugzilla.yoctoproject.org/show_bug.cgi?id=7880

Any questions, let me know.

Belén