support/graph-size: add option to change percentage to group in Others

support/graph-size: add option to change percentage to group in Others

Currently, we group packages that contribute less then 1%, into the
"Other" category.

However, in some cases, there can be a lot of very comparatively small
packages, and they may not exceed this limit, and so only the "Others"
category would be displayed, which is not nice.

Conversely, if there are a lot of packages, most of which only so
slightly exceeding this limit, then we get all of them in the graph,
which is not nice either.

Add a way for the developers to pass a different cut-off limit. As for
the dependency graph which has BR2_GRAPH_DEPS_OPTS, add the environment
variable BR2_GRAPH_SIZE_OPTS to carry those extra option (in preparation
for more to come, later).

Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
Cc: Thomas De Schampheleire <thomas.de_schampheleire@nokia.com>
[Arnout:
 - remove empty base class definition from Config;
 - use parser.error instead of ValueError for invalid argument.]
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>