How to build

1. Install yocto dependencies

sudo apt install -y chrpath diffstat g++ gawk gcc liblz4-tool make rpcbind

2. Git clone poky and meta-rapsberrypi

mkdir sources
cd sources
git clone https://github.com/yoctoproject/poky.git -b scarthgap
git clone https://github.com/agherzan/meta-raspberrypi.git -b scarthgap
cd ..

3. Enter poky

source sources/poky/oe-init-build-env build
cd ..

4. Add the raspberry pi layer to bblayers.conf
bblayers.conf

# POKY_BBLAYERS_CONF_VERSION is increased each time build/conf/bblayers.conf
# changes incompatibly
POKY_BBLAYERS_CONF_VERSION = "2"

SRCDIR := "${@os.path.abspath(os.path.dirname(d.getVar('FILE', True)) + '/../../sources')}"

BBLAYERS ?= " \
  ${SRCDIR}/poky/meta \
  ${SRCDIR}/poky/meta-poky \
  ${SRCDIR}/poky/meta-yocto-bsp \
  ${SRCDIR}/meta-raspberrypi \
  "

5. Set MACHINE in local.conf to one of the supported boards
The support boards are listed in meta-raspberrypi/conf/machine
build/conf/local.conf

...
MACHINE ??= "raspberrypi5"
...

6. Build
6.1 Trouble shooting before build

ERROR: Nothing RPROVIDES 'linux-firmware-rpidistro-bcm43455'

https://meta-raspberrypi.readthedocs.io/en/latest/ipcompliance.html#linux-firmware-rpidistro

6.2 Build

pushd build
bitbake core-image-base
popd

How to flash

Install bmaptools

sudo apt install bmap-tools

Flash the image

pushd build/tmp/deploy/images/raspberrypi5
sudo bmaptool copy core-image-base-raspberrypi5.rootfs-*.wic /dev/sdX
popd