Normalize temporary working directory to not fail with relative directory path

Normalize temporary working directory to not fail with relative directory path

When invoking "grml2is -t tmpdir ..." then the kernel and
initrd files receive a wrong leading directory path on the
resulting ISO, leading to a broken ISO. Since specifying
a relative path ("-t tmpdir" instead of "-t ${PWD}/tmpdir")
should be possible as well let's just normalize it.