Code in a functions.php (or any file loaded by that file) should not run automatically, eg. when the file is included. All code should be bound to hooks.
That means, you don’t need a special action in the parent theme. The child theme can turn off all parent theme callbacks with remove_action() and remove_filter().

But you can make that easier for child theme authors by adding a custom action after you have registered the callbacks.

Example

Parent theme

namespace ParentTheme;
// register callbacks
add_action( 'after_setup_theme', __NAMESPACE__ . '\setup_custom_background' );
add_action( 'after_setup_theme', __NAMESPACE__ . '\setup_custom_header' );
add_filter( 'body_class',        __NAMESPACE__ . '\enhance_body_class' );

// provide an entry point
do_action( 'parent_theme_loaded', __NAMESPACE__ );

Child theme

add_action( 'parent_theme_loaded', function( $namespace )
{
    remove_action( 'after_setup_theme', $namespace . '\setup_custom_header' );
});

See also this similar example for outdated PHP versions.

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If your parent theme ignores the principle of separation of concerns and mixes everything in just one or two setup functions, use a later priority and overwrite those calls:

Parent theme

add_action( 'after_setup_theme', 'some_setup_function' ); // priority = 10

Child theme

add_action( 'after_setup_theme', 'fix_theme_setup', 11 ); // priority = 11

function fix_theme_setup()
{
    remove_theme_support( 'post-formats' );
    // more changes here
}