[CentOS] Fixing grub/shim issue Centos 7

Mon Aug 3 17:24:37 UTC 2020
david <david at daku.org>

After trying several paths, some suggested on this list, here's my results.

1) Fixing a unbootable system wasn't practical in my 
case.  Fortunately, all my systems can be rebuilt from scratch.

2) When I was lucky enough to catch an updated system before reboot, 
backing out the defective updates wasn't possible.  Yum said there 
were no prior versions.

3) The most reliable method I found for Centos 7 was:
  - Re=install from scratch (luckily, my data files were safe and restorable)
  - Before running any updates, apply the fix suggested by Redhat and 
exclude updates to grub2, shim and mokutil.
  - Without the above 'exclude', the system became unbootable after a 
yum update even though the corrected versions of shim should have been loaded.

The system I'm dealing with is Centos 7.  I can easily rebuild it 
from scratch and test stuff without losing crucial data, if it would helpful.

4)  I haven't experimented yet with centos 8 because the hardware is 
remote and requires me to get a friend involved to help.  My local 
hardware is not supported by Centos 8, so it will remain on Centos 7 
until I replace the hardware or switch to a different Linux.