You cannot recreate the fastboot binaries, or get the source code for fastboot. These are based on proprietary code. If you want to use an open source bootloader you should look into ATF/UEFI. We will be switching snapshots for HiKey to use ATF/UEFI in the near future, in preparation for a release that uses an open source bootloader. Once that release is done, we will drop support for fastboot, and focus solely on ATF/UEFI, and possibly u-boot if time permits.