src/poky:overlayfs-etc: Unmount /sys and /proc before init

/sys filesystem mounted by the preinit script causes shadowing of
/sys/firmware/efi/ by double /sys mounting on systemd enabled systems
[1]. As the result EFI tooling is broken

[1]
sysfs on /sys type sysfs (rw,nosuid,nodev,noexec,relatime)
efivarfs on /sys/firmware/efi/efivars type efivarfs (rw,nosuid,nodev,noexec,relatime)
sysfs on /sys type sysfs (rw,relatime)

(From OE-Core rev: 94b3f86bac16ac3be468e23e1f6aad69cdf502d3)

Signed-off-by: Pavel Zhukov <pavelzhukoff.net>
Signed-off-by: Richard Purdie <richard.purdielinuxfoundation.org>

Author: landgraf