Single Binary with linux Kernel & filesystem

The Linux kernel is not setup to be run that way. The kernel separates memory into kernel space and user space. You can hack the kernel and have things built into it, but the kernel will look for the filesystem externally from itself in user space... if that makes any sense... I suck at wording things.

So, in a short form. not possible.

However, you could have a single file... just not an executable. You could make something like rootfs.ext2 which would be a single file with a file system inside of it, and a kernel inside of that, but you would have to loop mount the file somehow.

I could be wrong, but as far as I am aware you cannot merge the kernel and filesystem into a single executable.