Okay, figured things out myself 
Still not sure how are those files built and how do they get there, but proper files are included in /e/OS build ZIP file.
dtbo.img contains data about device’s hardware configuration and is used by the bootloader to initialize the correct hardware setup.
vendor_boot.img contains all the necessary drivers for the device and is used by the bootloader to load correct drivers.
So I suggest to extract those files from each /e/OS release, put on a /e/ foundation resource and link the resource in item 1 of “Flashing additional partitions” section. Or to replace this item with steps to extract those files manually:
- download latest release of payload-dumper-go to the same directory (*) where /e/OS build ZIP file is
- open terminal an
cdto (*) directory - extract payload:
unzip e-*-s-*-dev-lemonadep.zip payload.bin - extract payload-dumper-go:
tar xzvf payload-dumper-go_1.2.2_linux_amd64.tar.gz payload-dumper-go - run
./payload-dumper-go -partitions dtbo,vendor_boot -o . payload.bincommand — needed files will appear in (*) directory.