@niemeyer the boot-update interface goes beyond dtb updates, where do we stand with:
since @ondra solves exactly this bit in a rather elegant way while at the same time avoiding that porters do bootloader updates via uboot-shell hacks in uboot.env that we do not have any control over today …
his proposal is to limit the use of this interface to gadget (and/or kernel) snaps, it will simply move the update process to a more visible place into a snap upgrade hook from a u-boot hush-shell script and will avoid any potential issues that could be caused by the kernel vs u-boot FAT implementation (writes will happen from linux, not via the u-boot implementation with this).
i think @mvo will agree that this is a viable solution to the long standing problem of optional bootloader upgrades managed via the snap side.