[OE-core] How to use WIC to generate raw flash images

[Rick Altherr]

For OpenBMC (based on Yocto), the target is an SoC that has an external
boot flash connected via SPI.  Right now, we have a class (
https://github.com/openbmc/openbmc/blob/master/meta-phosphor/classes/image-overlay.bbclass)
used by our top-level image recipe (
https://github.com/openbmc/openbmc/blob/master/meta-phosphor/common/recipes-phosphor/images/obmc-phosphor-image.bb)
that manually constructs a flash image from the kernel, rootfs, etc.  I'd
like to move to an upstream-supported approach and I think that is WIC.
What I don't understand is:
- How do I build a raw flash image where it is divided into partitions but
has no partition table?
- How do I set padding to be 0xFF instead of 0x00?
- How do I include u-boot in the image?

Do I need to add a new imager plugin to support writing raw images with
non-zero padding and no partition table?  For U-Boot, do I add a new
bootimg plugin?

Rick
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