graphs: support generating png graphs

PDF files can not be easily embedded in other documents (eg. ODT, or HTML).

Add support for generating PNG graphs, by setting the GRAPH_OUT=pdf|png on
the command line:
    make GRAPH_OUT=png graph-build graph-depends

The default is still to generate PDF 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>