If your variable have a constant value or is set once value
is the right choice.
You can define it like this:
app = angular.module('myApp', []);
app.value('$webroot', 'localhost/webroot/app');
Now you can inject the service to your controller and use it:
app.controller('myController', ['$scope', '$webroot', function($scope, $webroot) {
$scope.webroot = $webroot;
}]);
Edit #1
To fit your updated question: You can use a constant the same way as a value:
app = angular.module('myApp', []);
app.constant('$webroot', 'localhost/webroot/app');
app.controller('myController', ['$scope', '$webroot', function($scope, $webroot) {
$scope.webroot = $webroot;
}]);