Append lists when merging conditional statements (bst-1)

It can happen that in the early stages, we need to process include files before having resolved the project options to do so, this only appears to happen when processing project.conf

This patch will enhance _yaml.py such that it will handle list composition specially when compositing the conditional statements, such that a composited conditional statement will be appended to the target list instead of overwriting it completely.

This is a backport of !2002 (merged)