@neil I'm not 100% sure about mainline Linux kernel, at least not with full features, but the MNT Pocket Reform runs a near-stock Debian by default. https://shop.mntre.com/products/mnt-pocket-reform
And @mntmn is very good about providing all the scripts to make the image builds easy to replicate, e.g. https://source.mnt.re/reform/reform-system-image/-/tree/main?ref_type=heads