support/scripts/graph-depends: use colors instead of colours

The graph-depends was not very consistent in colors vs. colours: some
parts were using colours, some parts were using colors.

Let's settle on the US spelling, colors.

This change the user-visble option --colours to --colors, but it is
unlikely that a lot of users customize the colors through
BR2_GRAPH_DEPS_OPTS, so this user interface change is considered
reasonable.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>