[Arm-dev] rainbow screen after "yum update" on 5/18/2018

I just updated my Centos7 Pi3 "yum update" on 5/18/2018, and now I am
facing a rainbow screen.  Red pwr light is nice and strong, even changed
power supplies.  Green ACK light blinks 2 times and stays off.  Screen is
stuck on the rainbow - no activity, nothing, with or without the USB drive,
nothing.  USB drive does not power up either - nothing.  Tried pressing the
keys 1 to 4 at power up, then any key on multiple pwr ups, no effect.
Found a similar post on https://www.raspberrypi.org/forums, search
"raspberry pi3 stuck on rainbow screen after Jessie update".

Removed micro card and read files on another machine

Without listing all the files individually, kernel7.img, the dtb files
bcm2708-rpi-xxx.dtb, bcm2710-rpi-xxx.dtb, bcm2835-rpi-xxx.dtb,
bcm2836-rpi-xxx.dtb, gcm2837-rpi-xxx.dtb.  All are dated 5/18/2018, the day
of my yum update.

The kernel is kernel-4.14.27-v7.1.e7.img dated 3/19/2018.

Other files bookcode.bin, fixup*.*, LICENCE.broadcom, start*.elf,
COPYING.linux all date 3/16/2018.

A file initrd-plymouth.img dates 2/26/2018.

I also see what look like older kernel files:  kernel-4.9.75-v7.1.el7.img -
1/9/2018; kernel-4.9.70-v7.1.el7.img - 12/19/2017;
kernel-4.9.64-v7.1.el7.img - 11/24/2017; kernel-4.4.26-v7.1.el7.img -
10/22/2016.

Any ideas on how to downgrade to previous kernel or how to fix this?  Can't
get into single user mode - pressing keys on pwr up has no effect.  I don't
know enough about the kernel and the boot process to start messing with
them.  I am stuck.
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