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'

docs/manual/rebuilding-packages.txt

@@ -15,6 +15,10 @@ The easiest way to rebuild a single package from scratch is to remove
 its build directory in +output/build+. Buildroot will then re-extract,
 re-configure, re-compile and re-install this package from scratch.
 
+For convenience, most packages support the special make targets
+<package>-reconfigure and <package>-rebuild to repeat the configure
+and build steps.
+
 However, if you don't want to rebuild the package completely from
 scratch, a better understanding of the Buildroot internals is
 needed. Internally, to keep track of which steps have been done and