Following this RPi U-Boot Howto.

I setup a cross compile chroot to build u-boot for my Raspberry 1:

sudo debootstrap sid sid
sudo chroot sid
dpkg --add-architecture armhf
apt-get update
apt install git-core gcc-arm-linux-gnueabi
cd /tmp
git clone git://git.denx.de/u-boot.git
cd u-boot
export CROSS_COMPILE=arm-linux-gnueabi-
make rpi_defconfig
make -j5 -s

Download the latest Jessie Lite Image and wrote it to my SDCARD (/dev/sdb in my case):

cd /tmp

wget https://downloads.raspberrypi.org/raspbian_lite_latest -O raspbian_lite_latest.zip
unzip raspbian_lite_latest.zip
dd if=2016-05-27-raspbian-jessie-lite.img of=/dev/sdb bs=1M

Setup NFS root, copy the Kernel to the tftproot and install u-boot:

cd /tmp
mkdir pi

mount /dev/sdb2 pi
mount /dev/sdb1 pi/boot

mkdir  /var/lib/tftpboot/pi
cp pi/boot/kernel.img /var/lib/tftpboot/pi/zImage
cp pi/boot/*.dtb /var/lib/tftpboot/pi/

mv pi/boot/kernel.img{,.old}
cp /tmp/sid/tmp/u-boot/uboot.bin pi/kernel.img

cp -a pi /mnt

umount pi/boot
umount pi

Setup NFS export:

/mnt/pi         10.0.0.1(rw,no_root_squash,async,no_subtree_check)

Configure dhcp:

host pi1 {
	hardware ethernet TH:AT:IS:MY:MA:C0;
	fixed-address 10.0.0.1;
	filename "/pi/boot.scr.uimg";
	option root-path "/mnt/pi";
}

Create the boot script that actually loads and boots the kernel /var/lib/tftpboot/pi/boot.scr:

setenv fdtfile /pi/bcm2708-rpi-b.dtb
setenv tftpblocksize 1024

usb start
tftp ${kernel_addr_r} /pi/zImage
tftp ${fdt_addr_r} ${fdtfile}

setenv bootargs earlyprintk console=ttyAMA0 console=tty1 ip=dhcp root=/dev/nfs rootwait smsc95xx.macaddr=TH:AT:IS:MY:MA:C0

bootz ${kernel_addr_r} - ${fdt_addr_r}

I needed to set my MAC as it would generate a new one on every boot.

Convert boot script to something u-boot can handle:

cd /var/lib/tftpboot/pi
mkimage -A arm -O linux -T script -C none -n boot.scr -d boot.scr boot.scr.uimg

Boot the Pi, interrupt u-boot and setup it’s environment:

setenv bootcmd 'dhcp; source ${fileaddr}'
saveenv
reset

I havn’t found a way to set config.txt options though.