change prefix for util-linux binaries from / to /usr

The prefix / causes executables like mount, umount, etc. to be placed
in /bin /sbin which therewith replace the busybox symlinks.
mount and umount e.g. are linked again libmount.so.

This is e.g. fatal during a sysupgrade, where /bin/mount,
/bin/umount, etc. but not libmount.so get copied into a ramdisk
which will result in non working executables within the ramdisk.

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@34791 3c298f89-4303-0410-b956-a3cf2f4a3e73