/boot content unlabeled

From investigating #1621 we also saw that /boot labels could be missing or wrong.

For example with podman bootc, I can see something like:

```
root@ibm-p8-kvm-03-guest-02:~# sudo restorecon -vnr /var/ /etc/ /usr/ /boot/
Would relabel /boot/ostree/default-181c7ce2b7abc61136e2caef56c8b05dcfdceef7f896926276a41b733658e58e/vmlinuz-6.17.0-0.rc5.42.fc43.x86_64 from system_u:object_r:usr_t:s0 to system_u:object_r:boot_t:s0
Would relabel /boot/ostree/default-181c7ce2b7abc61136e2caef56c8b05dcfdceef7f896926276a41b733658e58e/initramfs-6.17.0-0.rc5.42.fc43.x86_64.img from system_u:object_r:usr_t:s0 to system_u:object_r:boot_t:s0
Would relabel /boot/ostree/default-181c7ce2b7abc61136e2caef56c8b05dcfdceef7f896926276a41b733658e58e/.vmlinuz-6.17.0-0.rc5.42.fc43.x86_64.hmac from system_u:object_r:modules_object_t:s0 to system_u:object_r:boot_t:s0
```

This is likely a different bug from #1621 as we special case `/boot` content in bootc to support systems with SELinux disabled.