Here are some confirmed facts:
- initramfs updater looks only for lowercase update file names - "update*.bin"
- OTOH rootfs updater is case insensitive when it comes to update file names (that's why full updates begin with lowercase 'u' while incremental updates begin with uppercase 'U')
- initramfs updater silently fails if there are two or more "update*.bin" files and leaves the files as they are
- rootfs updater (the framework part to be more precise) deletes all but one the first one in ASCII order and then happily installs the leftover
- rootfs updater ignores zero-length "update files", doesn't even delete them
I'll utilize these facts in version 2 of this hack.