This is made possible by allowing to include a "%ARCH%" placeholder in the
environment variables DI_WWW_HOME and DI_DIR which is expanded to the correct
value at runtime.
Supported for Etch and later.
Only enable the rescue isolinux targets for etch.
I hope at least some of the etch stuff (like tools/boot/etch) can be used
for the daily CD builds that build using d-i daily builds.