Makefile: expose 'graph-depends' to generate a graph of the dependency tree

Generate the graph of the complete dependency tree by calling:
    make graph-depends

It's also possible to generate the graph-depends for a single package:
    make PKG-graph-depends

The graphs are generated in $(O)/graphs/

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>