Add missing type annotations

mypy only checks types in functions that have a return type annotation.

In `check_generated_files`, change from returning `None` to returning a
boolean, since the function is supposed to check some boolean-valued
assertion. So far, the function always returns `True`.

In `main`, explicitly return an `int` value as expected by the caller. When
calling `check_generated_files`, convert the boolean result into an exit
status.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>