Espressobin - building U-Boot

Continuing in the topic of the previous post - building the Kernel - I look into how Armbian builds U-Boot for the Espressobin.
There is some info on building U-Boot on the wiki but even though I tried to follow it to the letter, the U-Boot I build would never boot. Yet the U-Boot binaries build by Armbian work OK. Also note that there are several board revisions, so you certainly need sufficient new U-Boot or you can run into funny issues.

Espressobin - rebuilding kernel based on Armbian sources

Some time ago I bought the Espressobin board. It looked interesting but it turns out there is virtually zero support from the producer. There is some info on their wiki but it's quite obsolete.
I think the best way to get the Espressobin board running is installing Armbian. The good folks (or... the one person running the show?) build a decently working U-Boot and kernel and there is an SD card image that you can just dd onto a SD card. You don't even have to expand the partition to cover the whole SD card after copying the image -- after boot it expands it automatically.
This is all cool and dandy... But if you want to build your own U-Boot or kernel, Armbian does not make it easy. There is official documentation on how to build Armbian. This makes sense when you want to build the whole system but not so much if you just want to build the U-Boot and the kernel. Yet the fact that Armbian uses reasonably recent U-Boot and kernel, together with a bunch of patches, is interesting and I wanted to get to the U-Boot and kernel sources w/o having to run the whole Armbian build machinery. So I took a dive into the Armbian build scripts.