[DSE-Dev] Bug#757421: selinux-basics: selinux-activate doesn't install kernel args on extlinux

[undef]

I've just tested this on Mobian Sid on PinePhone. Selinux still doesn't 
seem to be configured by default with `selinux-basics` and 
`selinux-policy-default` installed.


Placing the below file in /usr/share/u-boot-menu/conf.d/zz-selinux.conf 
resolves the problem. Note the `zz-` prefix or one like it is mandatory 
as this must be processed after the Mobian config which will always 
overwrite the `U_BOOT_PARAMETERS` variable.

```

U_BOOT_PARAMETERS="${U_BOOT_PARAMETERS} selinux=1 security=selinux"
```


Thanks.