support/graph-depends: add option to specify output file

Currently, graph-depends outputs the dotfile program to stdout, and uses
stderr to trace the dependencies it is currently looking for.

Redirection was done because the output was directly piped into the dot
program to generate the final PDF/SVG/... dependency graph, but that
meant that an error in the graph-depends script was never caught
(because shell pipes only return the final command exit status, and an
empty dot program is perfectly valid so dot would not complain).

Add an option to tell graph-depends where to store the generated dot
program, and keep stdout as the default if not specified.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Samuel Martin <s.martin49@gmail.com>
[Thomas: rename metavar from DOT_FILE to OUT_FILE for consistency with
the rest of the new option naming.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>