As fallback/payload is compressed in bios.bin, I'm not sure what strings will provide. I did run strings on bios.bin (as well as looking at it in hex editor) and found nothing explicitly suggesting depthcharge (or u-boot for that matter) was the payload. I made assumption u-boot was payload based on presence of u-boot.dtb file in bios.bin.
u-boot.dtb is an ancient leftover that we haven't bothered to remove yet (or, rather, I think it might actually be gone from the latest boards... try a Kevin image). IIRC it actually contains the coreboot config or something weird? Anyway, red herring.
I welcome pointers to online documentation (either coreboot or chromiumos) that more clearly discuss coreboot inclusion within chromiumos build and development cycle.
Not much documentation unfortunately, in part because it keeps changing too often to be worth documenting. Your best bet is to just look at the ebuild sources directly:
https://chromium.googlesource.com/chromiumos/overlays/chromiumos-overlay/+/m... https://chromium.googlesource.com/chromiumos/overlays/chromiumos-overlay/+/m... https://chromium.googlesource.com/chromiumos/overlays/chromiumos-overlay/+/m... https://chromium.googlesource.com/chromiumos/overlays/chromiumos-overlay/+/m...
Hope you like portage, because that's all we have! ;)