With your help I think I got it now:

(function () {
    'use strict';

    var app = angular.module('sysMgrApp');

    app.directive('smFocus', [ '$timeout', function ($timeout, $parse) {
        return {
            restrict: 'A',

            link: function (scope, element, attribute) {
                var attribValue = attribute.smFocus;

                if (attribValue == '')
                    attribValue = true;
                else
                    attribValue = scope.$eval(attribValue);

                console.log('Element Id= ' + element[0].attributes["id"].value + ' attribValue=' + attribValue);
                if (attribValue== true) {
                    scope.$on('sm:focus', function () {
                        $timeout(function () {
                            element[0].focus();
                        }, 10);
                    });
                }
            }
        };
    }]);
})();

It is probably bad code, but seems to be working. If I have sm-focus it works and if I have it with the value of the variable, it seems to work as well (although I had to change the controller to define that variable earlier).