core: add a make target to check the dependencies

Add a make target that will checks the dependencies of all packages.
This will currently only detect circular dependencies, but more tests
can be added later if need be.

This can then be used in the autobuilders to automatically report
dependency issues.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>