How to deploy files to /boot partition with Yocto - linux

Question

I'm trying to deploy some binary files to /boot in a Yocto image for RPi CM3 but it deploys them to the wrong location.
do_install() {
install -d ${D}/boot/overlays
install -m 0664 ${WORKDIR}/*.dtb ${D}/boot/overlays/
install -m 0664 ${WORKDIR}/*.dtbo ${D}/boot/overlays/
}
The files are deployed to /boot in the / partition of the final image, but not to the /boot partition. So they are not available at boot time.
I already googled and studied the kernel recipes (and classes) of the Poky distribution but I didn't find the mechanism it uses how to ensure that the files are deployed to the boot image (and not to the /boot dir in the root image).
Any help is appreciated :)
Update #1
In my local.conf I did:
IMAGE_BOOT_FILES_append = " \
overlays/3dlab-nano-player.dtbo \
overlays/adau1977-adc.dtbo \
...
"
And in my rpi3-overlays.bb
do_deploy() {
install -d ${DEPLOYDIR}/${PN}
install -m 0664 ${WORKDIR}/*.dtb ${DEPLOYDIR}/${PN}
install -m 0664 ${WORKDIR}/*.dtbo ${DEPLOYDIR}/${PN}
touch ${DEPLOYDIR}/${PN}/${PN}-${PV}.stamp
}
Using this the image builds, but the files stillt don't get deployed in the /boot partition.
Using RPI_KERNEL_DEVICETREE_OVERLAYS I get a build error because the kernel recipe tries to build the dtbo files like dts files.

Answer 1

RPI images are created with sdimage-raspberrypi.wks WIC wks file. It contains:
part /boot --source bootimg-partition ...
so it uses bootimg-partition.py wic plugin to generate /boot partition. It copies every files defined by IMAGE_BOOT_FILES variable.
It seems you want to add some devicetree overlays, so you need to modify machine configuration and more specifically RPI_KERNEL_DEVICETREE_OVERLAYS variable. IMAGE_BOOT_FILES variable is set in rpi-base.inc.
If you don't have any custom machine or custom distro defined, you can add it in local.conf:
RPI_KERNEL_DEVICETREE_OVERLAYS_append = " <deploy-path>/<dto-path>"
You can see here how to add files in deploy directory.

Answer 2

After too many hours of investigation it turned out, that deploying files to other partitions than / is not easily possible. I now went the way of a post-processing script that mounts the final image, deploys the additional files and unmounts it.
# Ensure the first loopback device is free to use
sudo -n losetup -d /dev/loop0 || true
# Create a loopback device for the given image
sudo -n losetup -Pf ../deploy/images/bapi/ba.rootfs.rpi-sdimg
# Mount the loopback device
mkdir -p tmp
sudo -n mount /dev/loop0p1 tmp
# Deploy files
sudo -n cp -n ../../meta-ba-rpi-cm3/recipes-core/rpi3-overlays/files/* tmp/overlays/
sudo -n cp ../../conf/config.txt tmp/config.txt
sudo -n cp ../../conf/cmdline.txt tmp/cmdline.txt
# Unmount the image and free the loopback device
sudo -n umount tmp
sudo -n losetup -d /dev/loop0