Check the value of USE_OVERLAYFS

Hi,

whatever the value of USE_OVERLAYFS is, if it is uncommented, it will be considered as yes. I wanted to explicitly set it to no but the value actually doesn't matter since the only thing tested is if the variable is an empty string.

I choose to reset the value of the variable to an empty string if it is set to another value than yes in order to avoid multiple editing in the files. It's minimum editing but a bit of a hack maybe.

In this state it only supports "yes" as a value, but not the usual "yes|YES|y|Y|...".

ps: thx for the soft :)