Since those powerful chips are so damn cheap, maybe I can practice BGA routing and soldering with them? Maybe I can get a board made for about $50, and if it works, I can create my own BSP. I know for a fact that NXP mainlined the drivers for this chip, so it is very likely I can get stock Ubuntu or Debian armhf running on it.
1. You can, but not with $50 PCB. You need at least 4-mil process for 4-layer, or preferably 6-layer, unless you want to use plugged micro vias.
2. Routing dual 16-bit DDR is gonna be a great fun. When you bifurcate any high speed signal, things get rather nasty rather quickly.
3. Don't use mainline kernel and u-boot. Use NXP's own one. They have an official BSP if you just want to boot the chip, and a community one if you want to customize it. You will need basics on Yocto.
4. Prepare at least 16GB of RAM. Yocto for iMX6 fails to compile with 8GB if you don't have swap. It's one of the small things that have wasted me 2 days.
5. Yocto is not very popular in China, expect to ask no one and read a lot of materials in English. Prepare to buy books from abroad. Packt has a few books, one of them is specifically written for fsl-community-bsp (the BSP you will be using), the book is called "Embedded Linux Projects Using Yocto Project Cookbooks".