bitbake: toaster: Move project context variables to common scope

bitbake: toaster: Move project context variables to common scope

We have a bunch of context data which are used in multiple pages so it
makes more sense to have this in a single place libtoaster.ctx that's
accessible from each page rather than request it from every page.

(Bitbake rev: 4ef2774a2f683929c700550a9acc7b8f6074195b)

Signed-off-by: Michael Wood <michael.g.wood@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>