When doing a non-root user build, install gets run to populate symlinks.
The build is calling INSTALL_SYMLINK to do the dirty work.
This doesn't provide user/group information, so the entry in METALOG looks like this:
./sys type=link mode=0755 link=usr/src/sys
.. and makefs then populates it as the user in question from the filesystem, and you end up with this in the image:
root@raspberry-pi:~ # ls -l /sys
lrwxr-xr-x 1 adrian adrian 11 Dec 29 01:59 /sys -> usr/src/sys
(I added adrian:adrian to the raspberry pi image later, so the user/group resolves to the name. But it'll show up as 1001:1001 normally as that's my uid/gid.)