I think your best bet is to install Armbian, and use those resources listed in this thread.
While you could try and emulate piCore, you might even try getting familiar with how dCore works and starting from there. I suppose you could track the debian/ubuntu kernels offered up by Armbian rather than debian/ubuntu directly. Might make it easier. Many possibilities.
If you are only doing shell stuff, a real easy option is to install Armbian, and install and track the version of BusyBox that *core uses. Ie, download 1.27.2 of busybox (as of today, later versions are also available), do a make defconfig for busybox in Armbian, install it, and create the necessary symlinks (see the BB faq), and put it first in your path.
That's what I'm doing with my LePotato board until I get the necessary skills to transform it in a dCore-like version.