Ian, that is definitely one of the things that changed, and executing extlinux should fix your issue.
Maybe there is a timing problem between the installation of extlinux-bootloader and the new kernel, but it should only affect those who didn't have exlinux-bootloader installed.
Please add fdtdir /dtb-4.14.28-201.el7.centos.armv7hl and fdtdir /dtb-4.9.75-204.el7.centos.armv7hl to both kernels respectively.
If that works, I'll try to add a note to with some recommendations.

Thanks.
Pablo.

On 05/14/2018 07:18 PM, Pablo Sebastián Greco wrote:

Ian, that is not expected at all, especially not booting with the old kernel.
Since you still have the old contents, can you paste the contents of /boot/extlinux/extlinux.conf?

Sure.  I made a few changes to try to get the system to boot, but I'm
99% sure that this is what I had immediately after the 'yum update'.

#Created by RootFS Build Factory
ui menu.c32
menu autoboot centos
menu title centos Options
#menu hidden
timeout 60
totaltimeout 600

default=centos
label CentOS Linux (4.14.28-201.el7.centos.armv7hl) 7 (Core)
        kernel /vmlinuz-4.14.28-201.el7.centos.armv7hl
        append root=UUID=eb2c92c6-69cd-4a87-982d-900be79e928e LANG=en_US.UTF-8
        initrd /initramfs-4.14.28-201.el7.centos.armv7hl.img

label centos
        kernel /vmlinuz-4.9.75-204.el7.centos.armv7hl
        append root=UUID=eb2c92c6-69cd-4a87-982d-900be79e928e debug
        initrd /initramfs-4.9.75-204.el7.centos.armv7hl.img

The thing that jumps out is that the fdt/fdtdir entries are missing.

I actually did try adding an fdt line, but I missed the fact that it's
a directory, so that didn't work.  It's entirely possible that adding
the correct fdtdir line would have worked.

BTW, which BananaPi do you use? I've updated all my BPi-M1 without issues, but it is a rule for me to update in this order:
1) yum and rpm
2) all but kernel
3) kernel
To maybe that is why it didn't happen to me.

I believe that it's an M1, but I'm not 100% sure how to tell.  (It's
definitely a dual-core ARMv7 with 1GB.)