New firmware: 5.15.12

[Neil Brown]

I'm now using 5.15.12 on my gnubee.  The kernel requires a few patches to work, and as always they can be found in the gnubee/v5.15 branch of my 'linux' project on github

https://github.com/neilbrown/linux/tree/gnubee/v5.15

The config file and tools to build the firmware are in my gnubee-tools on github.

https://github.com/neilbrown/gnubee-tools/

Prebuilt images are on http://neil.brown.name/gnubee/

Apart from the kernel version update there are only small changes:

- nfsv4 is now enabled  (I don't know why it wasn't enabled before)

- the 'config' script now will install Debian "stable" instead of "stretch"

[lin...@gmail.com]

I was able to compile successfully with zram module added, and XFS removed. 

Thank you very much for your work!!!