MINOR: cfgcond: insert an expression between the condition and the term

MINOR: cfgcond: insert an expression between the condition and the term

Now evaluating a condition will rely on an expression (or an empty string),
and this expression will support ORing a sub-expression with another
optional expression. The sub-expressions ANDs a term with another optional
sub-expression. With this alone precedence between && and || is respected,
and the following expression:

     A && B && C || D || E && F || G

will naturally evaluate as:

     (A && B && C) || D || (E && F) || G