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 <pavel@zhukoff.net>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/files/overlayfs-etc-preinit.sh.in

@@ -48,5 +48,8 @@ else
     echo "PREINIT: Mounting </data> failed!"
 fi
 
+umount /sys
+umount /proc
+
 echo "PREINIT: done; starting </sbin/init>"
 exec {SBIN_INIT_NAME}