By searching for grmlenv.cfg we can make it more dynamic
instead of having to hardcode (hd0) which might be wrong
in the grml2usb situation.
There's also no need to set root/prefix multiple times, especially
since we depend on 'root=(hd0)' for the default ISO boot anyway.
Don't overwrite prefix if the grml_orig_prefix variable is empty.
Secure Boot enabled ISOs generated with grml-live >=0.31.0 are supported
with grml2usb >=0.16.0.