"grml-live -c GRMLBASE,GRML_SMALL,AMD64 -s jessie ..." fails while "grml-live -c
GRMLBASE,DEBIAN_TESTING,GRML_SMALL,AMD64 -s jessie" works fine.
This is caused because /etc/apt/sources.list is overwritten by the GRMLBASE
class and therefore no Debian mirror is available for installing aptitude, which
is no longer present  in debootstrapped chroots.

We either have to find a way how to install aptitude by default or otherwise
adjust documentation to make sure a proper DEBIAN_* class gets used by default.