Copy all files from /usr/${target} to /usr

It turns out there are actually several packages that install files to
$SYSROOT instead of $ROOT, not just udev rules.

build-rootfs.sh

@@ -65,10 +65,8 @@ fi
 
 cp -uv /usr/${target}/usr/bin/grub-editenv /mnt/gentoo/usr/bin/
 
-if [ -d /mnt/gentoo/usr/${target}/usr/lib/udev/rules.d ]; then
-    find /mnt/gentoo/usr/${target}/usr/lib/udev/rules.d \
-        -name '*.rules' \
-        -exec mv -t /mnt/gentoo/usr/lib/udev/rules.d/ {} +
+if [ -d /mnt/gentoo/usr/${target} ]; then
+    rsync -aO /mnt/gentoo/usr/${target}/ /mnt/gentoo/
 fi
 
 if [ -f /mnt/gentoo/etc/udev/hwdb.bin ]; then