support/graph-depends: add option to limit the depth of the graph

Currently, the complete dependency chain of a package is used to
generate the dependency graph. When this dependency chain is long,
the generated graph becomes almost unreadable.

However, it is often sufficient to get the first few levels of
dependency of a package.

Add a new variable BR2_GRAPH_DEPTH, that the user can set to limit
the depth of the dependency list.

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>