Moving to the Yocto Project, we can take a look at the core-image-minimal to identify its content and minimal requirements, as defined inside the Yocto Project. The core-image-minimal.bb image is available inside the meta/recipes-core/images directory, and this is how it looks:

SUMMARY = "A small image just capable of allowing a device to boot."

IMAGE_INSTALL = "packagegroup-core-boot ${ROOTFS_PKGMANAGE_BOOTSTRAP} ${CORE_IMAGE_EXTRA_INSTALL} ldd"

IMAGE_LINGUAS = " "

LICENSE = "MIT"

inherit core-image

IMAGE_ROOTFS_SIZE ?= "8192"

You can see here that this is similar to any other recipe. The image defines the LICENSE field and inherits a bbclass file, which defines its tasks. A short summary is used to describe it, and it is very different from normal package recipes. It does not have LIC_FILES_CHKSUM to check for licenses or a SRC_URI field, mostly because it does not need them. In return, the file defines the exact packages that should be contained in the root filesystem...