Here's a thought - with the vast number of SBC's out there, and each one desiring to put a version of piCore on it, this might be the easiest way as a developer:
Armbian supports many many non-rpi boards, and aside from their good work, you'll now have a native arm environment to develop this specific version of piCore in, reducing a lot of hassle.
https://www.armbian.com/So get Armbian up and running, and use those dev-tools from Armbian on your specific board to follow the build-scripts and whatnot to create piCore.
Might be the quickest and most convenient way to get piCore bootstrapped on board-X...