Move all files to /run/initramfs

Move all files to /run/initramfs

Given that we boot into a modern Linux distribution with the "/run" toplevel
directory, we can easily mount move the whole /run directory to the real
root in the end and have the complete initramfs later on in
/run/initramfs. All log files and /run states are still accessible and
to save space /run/initramfs can be removed later on.

Because the kernel does not mount a tmpfs on /run prior to unpacking the
initramfs cpio image, we have to copy ourselves very early to a tmpfs
and mount it on /run.
Due to lazy umount the old initramfs binaries should
be removed in the end by switch_root.

This feature can be turned on with "--prefix".