sim: Simplify verification methods

There are three methods that verify multiple images that essentially
manually implement the `all` and `any` methods of `Iterator`.  It makes
it difficult to understand what these do, as there is an early return of
true or false, with various negations of the check.

Replace these with calls directly to `any` or `all` so that it is
clearer what each function is doing.  The generated code should be the
same.

Signed-off-by: David Brown <david.brown@linaro.org>