support/graph-depends: don't show toolchain dependency for all packages

Al packages depend on 'toolchain'. Currently, 'graph-depends' graphs this
dependency. The resulting graph is thus cluttered with less-than-useful
information.

Instead, do not graph the 'toolchain' dependency for any package, save
for the fake 'all' package. The graph is now a bit more readable.

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>