Add Jam rule SuppressOptionalHaikuImagePackages as a mechanism to keep
packages from being installed.
Extend the UserBuildConfig.ReadMe document to cover the new command.
Closes ticket #6260.
Changes from v1:
* Simplified IsOptionalHaikuImagePackageAdded as suggested by Ingo.
* Added example as documentation.
git-svn-id: file:///srv/svn/repos/haiku/haiku/trunk@38100 a95241bf-73f2-0310-859d-f6bbb57e9c96