[package] add package source path override

[package] 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

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@16227 3c298f89-4303-0410-b956-a3cf2f4a3e73