x86_64: fix sysupgrade

Function move_config should be called after or during preinit_mount_root
hook in do_mount_root function. At this state sysupgrade.tgz is not in its
place during that time when do_mount_root is called. Function move_config
is called later so the sysupgrade.tgz stays in root directory to the second
restart when it is unpacked properly.

This patch adds move_config function to preinit_mount_root hook instead
of preinit_main and changes the filename of script to be called before
80_mount_root is called. It will prepare the sysupgrade archive for do_mount_root
within preinit_mount_root hook.

This issue was introduced for target x86. Since the target for x86_64 is
similar to x86, this issue is present also in this target. It solves the
ticket #15042 and #14088.

Signed-off-by: Jiri Slachta <slachta@cesnet.cz>
Tested-By: Stefan Hellermann <stefan@the2masters.de>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@39997 3c298f89-4303-0410-b956-a3cf2f4a3e73

target/linux/x86_64/base-files/lib/preinit/79_move_config

@@ -18,5 +18,5 @@ move_config() {
 	umount /mnt
 }
 
-boot_hook_add preinit_main move_config
+boot_hook_add preinit_mount_root move_config