[Buildroot] Buildroot way to populate additional partition

[Andrey Nechypurenko]

Hi Arnout,

Thanks for your suggestion!

>  There is not really a "Buildroot-way" to do this.

This is what I was suspecting. It is a little bit surprising for me because
I thought that having read-only rootfs with overlayfs for user data is a
rather typical solution for embedded systems. Maybe core Buildroot
developers can figure out how to better support this use-case in the
future ;-)

>  I've used two different approaches in the past:
>
> 1. In the post-build script, create the directories etc. in some temporary
> location and create the "data" filesystem by hand before calling genimage.
> Actually, if you don't use genimage.sh, you can simply use the --rootpath option
> of genimage to point to the data directory (so for the "data" image you would
> sent mountpoint to /).

I was also reading about --rootpath option and found out that it is hard to
use it with provided scripts. I am trying to avoid modifying existing
infrastructure as much as possible.

> 2. Create an init script to populate the data partition. This requires bypassing
> fstab and instead manually mounting the overlays as part of that init script.
> The advantage of this approach is that you can do "factory reset" by simply
> formatting the data partition.

This is a good point. Thanks for the hint.

Thanks,
Andrey.