According to the documentation, an empty file triggers firstboot
mode. Since we're not using a read-only root filesystem, we do not
need to leverage any other tricks here and we can just have no file
present.
KIWI currently configures grub2 slightly incorrectly,
such that installing new kernels does not correctly
switches the default kernel to the new one.
Work around this while we wait for a fix in KIWI
to deal with this problem.
Reference: https://github.com/OSInside/kiwi/issues/2198
