installkernel

installkernel version zImage System.map [directory]

installkernel installs a new kernel image onto the system from the Linux source tree. It is called by the Linux kernel makefiles when make install is invoked there. The new kernel is installed into {directory}/vmlinuz-{version}. If a symbolic link {directory}/vmlinuz already exists, it is refreshed by making a link from {directory}/vmlinuz to the new kernel, and the pre‐ viously installed kernel is available as {directory}/vmlinuz.old.

installkernel resides in /sbin only because the Linux kernel makefiles call it from there. It should really be in /usr/sbin. It isn't needed to boot a system.