WIC does this already for syslinux and grub bootloaders. I need more detailed info.
Reasigning to Igor to provide more details, or confirm that this is resolved.
u-bbot is the bootloader needed
u-boot, of course, typo
It turned out that U-Boot doesn't support mbr mode, since it has a different method to support non-EFI x86 devices. Closing this issue for now, I'll open a new one in case the method I mentioned requires support from wic.
Not a valid issue according to comment 5.