Stock Debian?

Will a netinstall image from debian.org work on a NanoPi M2?

I believe they modify the kernel configuration to work with their boards. 
For example, when I look at:

   make menuconfig

in linux-4.x.y supplied for the NanoPi I find USB_G_FRIENDLYARM

I searched for "Friendly" and only found that .. I expected to find more.

Do you see that in mainline?

Also, for the mini2440 the contents of /arm/arm/mach-s3c24xx/ was modified.

Check that file for your processor type and see if it has been changed,
both in their kernel and mainline.

Try searching for *.ini as I believe some product had a friendlyarm.ini

I'll have to check. I usually use BSD

...rather than Linux, so I should pull down the sources from both
repositories and do a recursive diff.

Sounds like a plan.  I have used diff once on Linux source code, so won't
be of much help.

When you find the different files maybe list them for us.