It's not really up to the kernel. The kernel and initial ramdisk are loaded into memory by the bootloader. Once you are in the ramdisk you can do pretty much wh

It's not really up to the kernel. The kernel and initial ramdisk are loaded into memory by the bootloader. Once you are in the ramdisk you can do pretty much whatever you want to mount the root partition; including using a user space file system. If you wanted to, you could skip switching to the "real" root filesystem, and run your system entirely out of the ramdisk. With EFI, you can even skip having a seperate boot-loader entirely, and combine the kernel, ramdisk, and an EFI shim into a single EFI executable.