Power & Source of Big Ideas

I'm trying to build U-Boot (from git.denx.de/u-boot.git). I basically did `make nanopi-r5s-rk3568_defconfig; make` and I'm getting the following message at the end: Image 'simple-bin' is missing external blobs and is non-functional: rockchip-tpl atf-bl31 /binman/simple-bin/mkimage/rockchip-tpl (rock...
Where's the boot information? Things like: - How to build U-Boot - How to install it (how it's found, in which order various devices are tried to find the cmd.boot file, ...). - What's supported by the U-Boot build (is it supposed to show something on HDMI, if not why not? Does it support USB keyboa...
I can't find anywhere in the wiki where the boot process is described. All I see is explanations of how to take someone else's premade image and maybe how to tweak it, but I'm looking for documentation about how it works, so as to know what are the constraints on eMMC or uSD partitioning, for exampl...