Add documentation for <pkg>-rebuild and <pkg>-reconfigure

Since these two special make targets are very useful but not yet
mentioned in the documentation I added them to the make help and
the manual.

Signed-off-by: Stephan Hoffmann <sho@reLinux.de>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>

Makefile

@@ -654,6 +654,8 @@ help:
 	@echo
 	@echo 'Build:'
 	@echo '  all                    - make world'
+	@echo '  <package>-rebuild      - force recompile <package>'
+	@echo '  <package>-reconfigure  - force reconfigure <package>'
 	@echo
 	@echo 'Configuration:'
 	@echo '  menuconfig             - interactive curses-based configurator'