The function do_symlink_kernsrc is not reentrant in the case where S is defined to a non-default value.

How to reproduce (tested with commit 7d06e4e30b39a901a3d0b263d5de43f954c3270b of poky which is quite close to master):

- add this line to meta/recipes-kernel/linux/linux-yocto_6.5.bb:
S = "${WORKDIR}/my-kernel-source-dir"

- build linux-yocto a first time:
bitbake linux-yocto

This works, and you can see that the function do_symlink_kernsrc created a symlink :
```
poky/build$ ls -l tmp/work-shared/qemux86-64/
total 8
drwxr-xr-x 2 ecordonnier ecordonnier 4096 Dec 15 13:18 kernel-build-artifacts
drwxr-xr-x 2 ecordonnier ecordonnier 4096 Dec 15 13:18 kernel-source


poky/build$ ls -l tmp/work/qemux86_64-poky-linux/linux-yocto/6.5.13+git/
total 1480
...
...
lrwxrwxrwx   1 ecordonnier ecordonnier     73 Dec 15 13:18 my-kernel-source-dir -> /home/ecordonnier/dev/poky/build/tmp/work-shared/qemux86-64/kernel-source
```

- Build a second time (in reality this happened to me when building again after updating poky to a version which modified kernel.bbclass):
bitbake -C symlink_kernsrc linux-yocto

This will fail, and leave the directory structure in a completely broken state:
kernel-source is now a symlink to itself, and my-kernel-source-dir is a broken symlink:

```
poky/build$ ls -l tmp/work-shared/qemux86-64/
total 8
drwxr-xr-x 2 ecordonnier ecordonnier 4096 Dec 15 13:18 kernel-build-artifacts
lrwxrwxrwx 1 ecordonnier ecordonnier   73 Dec 15 13:18 kernel-source -> /home/ecordonnier/dev/poky/build/tmp/work-shared/qemux86-64/kernel-source

poky/build$ ls -l tmp/work/qemux86_64-poky-linux/linux-yocto/6.5.13+git/
...
lrwxrwxrwx   1 ecordonnier ecordonnier     73 Dec 15 13:20 my-kernel-source-dir -> /home/ecordonnier/dev/poky/build/tmp/work-shared/qemux86-64/kernel-source

```