x86_64: fix invoking sysupgrade on PARTUUID mounted system

Fix sysupgrade to determine the correct boot block device to use if
/proc/cmdline contains a PARTUUID root.

Change the preinit move_config() hook to use the UUID scanning code
from sysupgrade in order to avoid code duplication.

Signed-off-by: Jo-Philipp Wich <jow@openwrt.org>

SVN-Revision: 42968