IMAGE PREREQS:
1. As kernel you can either compile https://github.com/raspberrypi/linux or use the debian binary http://archive.raspberrypi.org/debian/pool/main/r/raspberrypi-firmware/
2. You then need to make an initramfs for the nfsroot. I have used dracut as per https://wiki.gentoo.org/wiki/Dracut#NFS_boot
3. You need to edit the inittab file if you intend on using the serial console https://wiki.gentoo.org/wiki/Raspberry_Pi_3_64_bit_Install#Serial_port_configuration


# On the rpi.
1. Update rom and bootloader options and enable netboot if sd card is not present
- See https://www.raspberrypi.com/documentation/computers/raspberry-pi.html#updating-the-eeprom-configuration
[all]
BOOT_UART=1
BOOT_ORDER=0x21
DHCP_TIMEOUT=45000
DHCP_REQ_TIMEOUT=4000
TFTP_FILE_TIMEOUT=30000
TFTP_PREFIX=1
TFTP_PREFIX_STR=WHATEVERDIR/
WAKE_ON_GPIO=1
POWER_OFF_ON_HALT=0

# Setup TFTP + NFS Server
1. Install in.tftpd and nfs
- rc-update add in.tftpd
- rc-update add nfs

2. cp -a /boot into /var/tftpboot/WHATEVERDIR
3. edit /var/tftpboot/WHATEVERDIR/cmdline.txt
console=serial0,115200 root=/dev/nfs nfsroot=172.12.1.57:/WHATEVER/ROOTFS,vers=3,proto=tcp rw ip=dhcp rootwait
4. add /var/tftpboot/WHATEVERDIR to /etc/exports
/var/tftpboot/WHATEVERDIR *(fsid=1,rw,sync,no_subtree_check,no_root_squash)

# ROOTFS setup
1. cp -a / /WHATEVER/ROOTFS
2. edit /WHATEVER/ROOTFS/etc/fstab
172.12.1.1:/var/tftpboot/WHATEVERDIR /boot nfs noauto,noatime,vers=3,proto=tcp 1 2
3. add the roofs dir to /etc/exports
/WHATEVER/ROOTFS *(rw,sync,no_subtree_check,no_root_squash)