Controller-0 failed to install getting to grub during upgrade

[Samuel Presa Toledo]

Brief Description
-----------------
During upgrade from stx6.0 to stx7.0, controller-0 enters into grub terminal, and the upgrade cannot be conclude successfully. The root cause is the different place of pxeboot configuration between the two version:

stx7.0 is /var/pxeboot/pxelinux.cfg
stx6.0 is /opt/platform/config/21.12/pxelinux.cfg

When an upgrade from stx6.0 to stx7.0 is executed, the pxeboot configuration should be migrated from release N to N+1, and this modification was not considered for pxeboot configuration migration.

Severity
-----------------
Major.

Steps to reproduce
-----------------
Follow upgrade procedure from stx6.0 (SW_VERSION 21.12) to stx7.0 (SW_VERSION 22.06).

Expected Behavior
------------------
Controller-0 upgrade successfully.

Actual Behavior
----------------
Controller-0 upgrade failure and it enters into grub terminal

Reproducibility
----------------
this issue was observed in 3 different labs.

System Configuration
----------------
AIO-DX

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.opendev.org/c/starlingx/config/+/841163
Committed: https://opendev.org/starlingx/config/commit/2ba470a29f7f706aeb5889adfa4657356fc65b35
Submitter: "Zuul (22348)"
Branch: master

commit 2ba470a29f7f706aeb5889adfa4657356fc65b35
Author: Samuel Presa Toledo <email address hidden>
Date: Mon May 9 16:00:23 2022 -0400

    Adjust pxeboot config files path in an upgrade

    During the upgrade from stx6.0 to stx7.0, the content of
    /opt/platform/config/22.06/pxelinux.cfg/ is copied from the previous
    release. This way, the grub.cfg, which is a symlink, points to
    /pxeboot/pxelinux.cfg.files/grub.cfg. As in release stx7.0
    /pxeboot was moved to /var/pxeboot, the symlink should be recreated
    to point the correct path, which is
    /var/pxeboot/pxelinux.cfg.files/grub.cfg. This fix recriates the symlink
    pointing to the correct path.

    Note:
     cert-manager
     nginx-ingress-controller
     oidc-auth-apps
     platform-integ-apps

     The above apps were removed from upgrade path on 65-k8s-app-upgrade.sh
     script to finish upgrade process successfully

    Test plan
      PASS: Run initial bootstrap successfully
      PASS: Upgrade AIO-DX from stx6.0 to stx7.0

    Closes-bug:1972572

    Signed-off-by: Samuel Presa Toledo <email address hidden>
    Change-Id: Icb2b026ccac583295089e922e98d2acd207cabdb