Copy file into bootfs partition

Using the Distribution Package for STM32MP1 I wrote a simple function to copy some file to the rootfs image and execute fine-tuning commands:

my_postprocess_function() {
	cp -dr /home/osboxes/Documents/overlay/* ${IMAGE_ROOTFS}
	ln -s /lib/systemd/system/apache2.service ${IMAGE_ROOTFS}/etc/systemd/system/multi-user.target.wants/apache2.service
}
 
ROOTFS_POSTPROCESS_COMMAND += "my_postprocess_function; "

It works fine but of course don't work for files in `/boot` because it's another partition and the tree under `${IMAGE_ROOTFS}` does not contain it.

So, what's the right syntax to execute any command in the `boot/` directory?