graph-depends: add option to pass arbitrary dot options

Kids nowaday seem to prefer a left-to-right drawing rather than the
more conventional and historical top-down drawing.

Rather than multiply the number of environment variables, just add
a single one where the user can pass arbitrary dot options, such as:

    make BR2_GRAPH_DOT_OPTS=-Grankdir=LR graph-depends

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Maxime Hadjinlian <maxime.hadjinlian@gmail.com>
Cc: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>