initramfs: switch to tmpfs to fix ujail

initramfs: switch to tmpfs to fix ujail

Hauke wrote:
> We want to run some processes in the procd-ujail, this works when we
> use a SquashFS image and an overlay file system, but when we use an
> initramfs it does not work.
> [...]
> When we switch from initramfs to tmpfs, it is working, we added this
> code to target/linux/generic/other-files/init to make [it] work.

Move files to newly mounted tmpfs and then use switch_root to chroot
into new rootfs and free initramfs.

Suggested-by: Hauke Mehrtens <hauke@hauke-m.de>
Signed-off-by: Daniel Golle <daniel@makrotopia.org>