Add new dependency scope "bootstrap" to support concept of boot class path

When it comes to cross-platform compilation, one needs to feed in a bootclasspath into various JDK tools like javac or javadoc (see MCOMPILER-20, MOJO-606). Currently, the affected Maven plugins provide a configuration parameter on their own to deal with this concept. Usually, this configuration needs to be configured with file system paths (either to some JARs in the local repo or to the user's JDK installation).

That's not really the Maven way. Maven builds classpaths from dependencies and hence, the bootclasspath should be configurable via dependencies, too. Having a dedicated scope for such dependencies would make plugin development and configuration more straightforeward in the case of cross-platform compilation.

My post MCOMPILER-20#action_112355 may be used as an inspiration, how such a new scope could look like.