Update netboot.xyz.lkrn to v2.0.19, supporting Grml 2020.06 release

Update netboot.xyz.lkrn to v2.0.19, supporting Grml 2020.06 release

Retrieved via:

  % wget -q https://github.com/netbootxyz/netboot.xyz/releases/download/2.0.19/netboot.xyz.efi
  % wget -q https://github.com/netbootxyz/netboot.xyz/releases/download/2.0.19/netboot.xyz.lkrn
  % wget -q https://github.com/netbootxyz/netboot.xyz/releases/download/2.0.19/netboot.xyz-sha256-checksums.txt
  % sha256sum -c netboot.xyz-sha256-checksums.txt 2>/dev/null | grep -e 'netboot.xyz.efi:' -e 'netboot.xyz.lkrn:'
  netboot.xyz.efi: OK
  netboot.xyz.lkrn: OK

See https://github.com/netbootxyz/netboot.xyz/issues/678