Checkout other layers:
poky
meta-openembedded
meta-raspberrypi

Source oe-init-build-env and use config files in this repo
TEMPLATECONF=meta-burak/conf/samples/ . poky/oe-init-build-env

Alternative, if TEMPLATECONF environment variable is not working:
Edit poky/.templateconf file. It should have the full path of the conf files, e.g.
TEMPLATECONF="/home/burak/yocto/meta-burak/conf/samples"

To build bootable base image:
MACHINE=raspberrypi3-64-burak bitbake core-image-base

Write to sd card with:
hardknot:
sudo bmaptool copy core-image-base-raspberrypi3-64.wic.bz2 /dev/mmcblk0

zeus:
sudo dd if=core-image-base-raspberrypi3-64.rootfs.rpi-sdimg of=/dev/mmcblk0 bs=16M


You can easaly update u-boot by mounting the boot partition (/dev/mmcblk0p1) and replacing
kernel8.img (64 bit) with u-boot.bin. 
For details see here:
https://github.com/ARM-software/arm-trusted-firmware/blob/master/docs/plat/rpi3.rst