Corona: make browser support variables available for extensions

In Corona the browser support variables, `legacy-support-for-ie*` are declared in the `_variables.scss` partial file and imported after all extensions. So they are not available for them.

For instance the `box-sizing` mixin in toolkit is never printing the polyfill.  

With the following support set:
<pre>
$legacy-support-for-ie6: true;
$legacy-support-for-ie7: true;
</pre>
 You always get:
<pre>
*, *:after, *:before {
  -webkit-box-sizing: border-box;
  -moz-box-sizing: border-box;
  box-sizing: border-box;
}
</pre>
Instead of:
<pre>
*, *:after, *:before {
  -webkit-box-sizing: border-box;
  -moz-box-sizing: border-box;
  box-sizing: border-box;
  *behavior: url('../behaviors/box-sizing/boxsizing.php');
}
</pre>