> How is the image made bootable in an unprivileged build ? > The same can probably be used to change the boot option.
installboot(8) against a single raw ffs partition, then MBR and swap etc. for the whole image are concatenated by cat(1): https://nxr.netbsd.org/xref/src/distrib/common/bootimage/Makefile.bootimage#371 (yes, it's so ugly, but no other better suggestion in PR/45153) --- Izumi Tsutsui
