graph-depends: rename the variable to pass the max depth

There will soon be new options to the graph-depends script, which we
can only sanely pass via environment variables.

Currently, we use such an environment variable to pass the maximum depth
of the dependency graph; the name of that variable is explicit that it
contains just the depth.

However, there has been so far no release of Buildroot which would make
use of that variable, so no user should have come to rely on it.

Rename that variable so it is less specific, and more generic, so it can
be used to pass more options to graph-depends.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Maxime Hadjinlian <maxime.hadjinlian@gmail.com>
Cc: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>