Amlogic does not provide sources for the firmware and tools needed to create a
bootloader image so it is necessary to use some proprietary binaries. This is
described in u-boot-amlogic documentation (cf. doc/board/amlogic/libretech-cc.rst
in u-boot sources).

The libretech-cc's u-boot.bin file from u-boot-amlogic package installed on the
target image, and used to create the bootloader is automatically extracted
during target image build.