Copy etc files to tmpfs and mount on etc
Copy etc files to tmpfs and mount on etc

This change solves the problem of readonly initramfs updating
the etc files. The init process copies the etc directory to
/tmp/etc and bind mount to /etc.

Tested:
```
~/# mount
none / rootfs rw 0 0
devtmpfs /dev devtmpfs rw,relatime 0 0
proc /proc proc rw,relatime 0 0
none /tmp tmpfs rw,relatime 0 0
devpts /dev/pts devpts rw,relatime,gid=0,mode=620,ptmxmode=666 0 0
none /dev/shm tmpfs rw,relatime 0 0
sysfs /sys sysfs rw,relatime 0 0
none /etc tmpfs rw,relatime 0 0
~/# echo hihi >/tmp/etc/hello
~/# cat /etc/hello
hihi
```

Signed-off-by: Medicine Yeh <ce0f0ca58c98e036a28df53229ac3ccdc6eee9c7@gmail.com>

u-root/u-root,u-root/u-root,u-root/u-root