extlinux-conf-builder: Fix warning when building in chroot

When using extlinux-conf-builder in a nix build using chroots, the
following error message could be seen:

/nix/store/XXX-extlinux-conf-builder.sh: line 121: cd: /nix/var/nix/profiles: No such file or directory

To avoid this, just skip the code path parsing /nix/var/nix/profiles
when $numGenerations (passed from the command line) is 0 (which is the
only legal value of $numGenerations in a nix build context).