Fixup source dependancies

 * Add the builder git index and included configs to a packages source
   dependancies.
Allow packages to define EXTRA_SOURCES

 * Some packages rely on config files and such out of ${F} which are copied
   into ${S} for some purpose.  These files are not normally caught as a
   dependancy.  Now we can define these files in EXTRA_SOURCES so that builder
   will know to rebuild a package if these files change.