Remove support for shared configuration cache

The configuration cache shared between packages, while being in
principle a nice idea to speed-up the configuration of packages by
avoiding repetitive identical checks, turned out to be unreliable due
to the subtle differences between similar but not identical checks in
different packages. After spending some time trying to fix those, we
concluded that supporting the shared configuration cache is definitely
too hard and too unreliable, and that we'd better get rid of it
altogether.

This patch therefore removes the shared configuration cache
infrastructure and usage.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>