fix for overriding MFS_HDD_CONFIG by static content

[wfr3d]

I think I found a bug in the code. Regardless of what we pass in the MFS_HDD_CONFIG environment variable, the value is overwritten by the "echo" command on lines 29 or 31. My solution: when the MFS_HDD_CONFIG variable is used, it leaves the user fully responsible for the content of the mfshdd.cfg file.