Provide `simple-init` just mounting and kexec’ing Linux kernel

Some user, do not want the security features of Heads, but only the environment to build the Linux kernel and assemble an initrd. A simple init should be provided, which has only a time-out, and will boot the configured Linux kernel, or, if interrupted, provide a shell.

No file or test is named. Start by locating the existing init entry point and the configured Linux-kernel and initrd boot path; done means a simple init provides a timeout, mounts the required environment, boots the configured kernel with kexec, and offers a shell when interrupted.

Written by the indexing model from the issue text.