Enabling BR2_PACKAGE_RPI_FIRMWARE_INSTALL_DTB_OVERLAYS from Target Packages -> Hardware Handling -> Firmware does not copy *.dtbo files into boot/overlays folder.
Artem, Please compare board/raspberrypi/genimage-raspberrypi.cfg and board/raspberrypi3/genimage-raspberrypi3.cfg . You'll see how to add the overlays .dtbo files for your specific rpi version. The configuration files provided in Buildroot are only basic configs; you have to adapt them to your own needs. Regards, Yann E. MORIN.