support/scripts/graph-depends: add --flat-list option

support/scripts/graph-depends: add --flat-list option

graph-depends currently spits out a graph in .dot format. However, as
part of the upcoming introduction of <pkg>-show-recursive-depends and
<pkg>-show-recursive-rdepends, we need graph-depends to be able to
display a flat list.

Signed-off-by: George Redivo <george.redivo@datacom.ind.br>
[Thomas:
 - Rebase on top of graph-depends changes
 - Do not display the package name itself in the list, only its
   dependencies (or reverse dependencies)
 - Display the result on a single line, instead of one package per
   line, in order to match what <pkg>-show-depends and
   <pkg>-show-rdepends are doing today.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>