Experiment with u-root-builder produced u-root releases (to expand and easier PoC)

[tlaurion]

https://github.com/linuxboot/u-root-builder now builds x86_64 compatible initramfs which could be applied atop Heads initramfs.

Once #1384 is merged, some experiments could happen to have u-root downloaded from that project/somewhere else and applied on top of Heads in tmpfs/ramfs/squashfs on-demand and resume init from u-root.