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Specification
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- exit as not applicable unless ppc64 architecture is used

- Log error message and exit with ERROR when:
  - cannot detect architecture (very improbable, do not need test)

NEEDS_ACTION
(if... then...)
- the yaboot RPM is not installed (unsupported solution for ppc64
  architecture) or it is not signed by Red Hat
    - inform user that yaboot signed by RH has to be installed and
      used for booting of the system + log high risk
- the /etc/default/grub doesn't exists yet:
    - generate [1] for the grub2 (should be correct at least for basic/default
      installation and partitioning)
    - inform user that content of the file has to be checked and then moved
      into the /etc/default/grub
    - high risk
- the /etc/default/grub exists:
    - back up the file into [1] (+ log info)
    - inform user that validity of the file should be check
    - high risk

In addition the pre-upgrade script [0] must exists, which will be processed
by r-u-t and which exits with 1 when:
  - yaboot rpm is not installed or it is not signed by Red Hat
    (note: ignores activation of the devel_mode)
  - the /etc/default/grub doesn't exist or it is not regular file

[0] /root/preupgrade/preupgrade-scripts/yaboot/yaboot_check
[1] /root/preupgrade/dirtyconf/etc/default/grub
