add package source path override - use external source tree instead of source ball...

add package source path override - use external source tree instead of source ball - the external package source tree will be included as symlink - make package/<name>/clean will delete the symlink instead of whole source tree - usefull in conjunction with external SCM like ClearCase - package-version-override.mk has to be included before package.mk

SVN-Revision: 16227