[-][FO] _PS_THEME_MOBILE_OVERRIDE_DIR_ not defined

When there is no mobile theme provided with the current theme, the variable _PS_THEME_MOBILE_OVERRIDE_DIR_ is not defined. We so get the following error in apache logs:

"PHP Notice:  Use of undefined constant _PS_THEME_MOBILE_OVERRIDE_DIR_ - assumed '_PS_THEME_MOBILE_OVERRIDE_DIR_' in FrontController.php"
Merge pull request #975 from ldecoker/patch-10

[-] FO : _PS_THEME_MOBILE_OVERRIDE_DIR_ not defined