Secure Boot GRUB template: be more dynamic + don't overwrite with empty variable

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.