support/graph-depends: add option to exclude mandatory deps

support/graph-depends: add option to exclude mandatory deps

Some times, multiple dependency graphs for a set of packages (mostly
the application-level packages for the project) are included in reports
(e.g. delivery notes). Repeating the mandatory dependencies on all
those graphs is useless and clutters the important dependencies.

When we had only two such mandatory dependencies (toolchain, skeleton),
it was manageable to list them as manual exclusions:
    -x toolchain -x skeleton

But we now have quite a few such dependencies, and it becomes a bit more
cumbersome to manage, not counting the ones we may add in the future.

Add an option to exclude all those mandatory dependencies, to generate
neat graphs.

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