Added fallback elf launch

[marco-calautti]

There is a subtle case when there is a valid = entry but the path on the right side is wrong and unrecoverable. For example, a=wiiu//apps/..., in this case Haxchi will recognize at least a valid button mapping, and will always try to run the associated path when pushing the button, but will fail. In this case, the fallback to HBL will never have a chance to work thus making Haxchi not able to boot anything.

Altough this might be a problem in a coldboothax context, I think it is a mild change which cannot hurt the master branch. But, as usual, I am providing this for your convenience.

cheers