On 09.06.19 20:53, Matt B wrote:
It is possible through u-root support for multiboot images  to chainload grub?
Yes, I would think so. But in case we are still on topic: It won't help you to boot Windows (unless you also implement UEFI services in your LinuxBoot and use a UEFI GRUB).
To chainload something for Windows I would currently go either one of these ways:
coreboot -> LinuxBoot -> SeaBIOS -> Windows loader coreboot -> LinuxBoot -> tianocore -> Windows loader
I think SeaBIOS already has an option to build a multiboot image. In either case you could also (in theory) pack either into a bzImage and feed that to kexec.