[yocto] Deploy rootfs image within another rootfs image

[Craig McQueen]

I've got two scenarios where I want to put a rootfs image within another rootfs image.

One scenario is for factory programming: My target is a BeagleBone Black type of system. I want to make a simple programmer to boot from SD card, which will partition and format the on-board eMMC and then write the bootloaders onto one partitions, and a rootfs image onto the other partition. So I want to make a recipe to build the programmer image, which contains another previously-built rootfs image at say /lib/firmware/rootfs.tar.gz. 

Another scenario is to implement firmware upgrade in-the-field. I am considering making a rootfs as a read-only SquashFS image. That image would then be put into a writable ext4 filesystem. An initramfs would use OverlayFS to mount the writable ext4 filesystem over the SquashFS image that is loop-back mounted.

What is necessary to make a Yocto recipe to make an image, that will build another image, and then copy its deployed .tar.gz image into its own rootfs image?

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Craig McQueen