make: add clean-pkg target (cleaning extracted & patched pkg sources)

Makefile.include

@@ -309,6 +309,9 @@ clean-intermediates:
 	-@for i in $(USEPKG) ; do "$(MAKE)" -C $(RIOTPKG)/$$i clean ; done
 	-@rm -rf $(BINDIR)/*.a $(BINDIR)/*/
 
+clean-pkg:
+	-@for i in $(USEPKG) ; do "$(MAKE)" -C $(RIOTPKG)/$$i distclean ; done
+
 distclean:
 	-@for i in $(USEPKG) ; do "$(MAKE)" -C $(RIOTPKG)/$$i distclean ; done
 	-@rm -rf $(BINDIRBASE)