core: catch failures in graph-depends

Currently, we generate the dependency graph in a single command, piping
the stdout of support/scripts/.graph-depends to the stdin of dot.

Unfortunately, this means we can't catch a failure of graph-depends, as
the shell can only treturn the exit code of the last command in a pipe.

Still, we do want to keep the output of graph-depends, and we in fact do
keep it by mean of a tee.

graph-depends has just gained the ability to generate its output to a
file, so we break the pipe in two differnet commands, so we can bail out
on graph-depends errors.

Do that for the two call sites.

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>