bin/element-info accesses it's library via a symlink:
bin/diskimage_builder -> diskimage_builder
This causes the relative path in elements.py to be
incorrect.
element-info silently fails in this situation, because
it allows missing 'element-deps' files, so that these files
may be optional.
This change causes element-info to fail explicitly if $ELEMENTS_DIR
is not set, as it now is when called by diskimage-create, and
adds tests reflecting this behavior.
Change-Id: Iec50f934feb13dfed64d69297a3af6ac9f842677
6cfea30246