The idea of NBoot is to hide all the low-level stuff from users so that they can concentrate on U-Boot and Linux and do not have to bother about DRAM settings, board configurations and firmwares. So NBoot is basically meant to be compiled by F&S, not by customers. But I can understand the reason for your need to do this. Maybe you can do your modifications in a rather generic way. Then when you are done, and if it's OK for you, we can add your improvements to our regular NBoot. This would save you some work in the future (see below).
To compile NBoot, you just have to rename bl31.bin from the ATF build to bl31-imx8mp.bin and store it to board/F+S/NXP-Firmware. Please note that we have a slightly modified version of ATF, so start with our github version, not the original one from NXP.
Then copy the DRAM-FW/Training data to this directory, too. They are part of the firmware-imx package. from NXP.
For a regular NBoot, you won't need TEE. A Trusted Execution Environment without Secure Boot does not make much sense.
Of course U-Boot must be configured for fsimx8mp, i.e. make fsimx8mp_defconfig. Then simply call make nboot. This will compile most of U-Boot, then SPL, then all the other sub-images (configurations, DRAM timings, etc:) and finally assembles the NBoot image. Please change NBOOT_VERSION in board/F+S/fsimx8mp/nboot/Makefile to make it different to our releases, e.g. add a prefix or suffix or similar.
Then install NBoot with fsimage save in U-Boot. Also be aware that if your modifications cause a failure of ATF, so that U-Boot is not started, you have to re-install the software using the UUU tool or our Recovery Tool.
We are currently in the process of writing a documentation for NBoot that will show all the internal details. But this takes a few more days until it is finished. It will only be availbale on request, so leave me a message if you want it.
Please note that using your own NBoot has some severe disadvantages. NBoot always handles DRAM and flash initialization. So for example, if a DRAM chip goes end-of-life and we replace it with a different one, we will most probably need new DRAM timings and therefore a new board configuration to support this. So all new boards that are shipped from then on, have the new NBoot preinstalled. But if you replace this version with your own old version, then booting will most probably fail, because your version does not know about the new DRAM timings and board-config yet. In other words, for every change that we do in NBoot, you also have to rebuild your own version of NBoot. This will result in quite a lot more work that you have to do. If you also change stuff in NBoot other than ATF, things may get even more complicated.
Your F&S Support Team
