build: support building update sites for any feature

e.g. with a command like this:
./build.sh site <feature-name>

Change-Id: Ic9ed15b75830d7e8d9b9e3a2555b670b8330289f
Reviewed-on: http://gerrit.esdi-community.eu/3151
Tested-by: Jenkins <sitemple@igd.fraunhofer.de>
Reviewed-by: Simon Templer <simon.templer@igd.fraunhofer.de>