graph-depends: refactor with more colors

Do not use the same colors for toolchain, host and target packages.

Signed-off-by: Francois Perrad <francois.perrad@gadz.org>
[yann.morin.1998@free.fr rephrase commit log]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>