cmdlib.sh: cleanup tmp_overridesdir between runs

In 230139d we started copying in `src/config/live` into an overlay to be
stored under `/usr/share/coreos-assembler/live` in our resulting images.
The way this is currently implemented yields a nested `live/live` folder
and duplicate contents getting created becuase we don't clean up between
runs:

```
/srv/tmp/override/usr-share-cosa/usr/share/coreos-assembler/live/README-devel.md
/srv/tmp/override/usr-share-cosa/usr/share/coreos-assembler/live/zipl.prm
/srv/tmp/override/usr-share-cosa/usr/share/coreos-assembler/live/isolinux
/srv/tmp/override/usr-share-cosa/usr/share/coreos-assembler/live/isolinux/isolinux.cfg
/srv/tmp/override/usr-share-cosa/usr/share/coreos-assembler/live/isolinux/boot.msg
/srv/tmp/override/usr-share-cosa/usr/share/coreos-assembler/live/EFI
/srv/tmp/override/usr-share-cosa/usr/share/coreos-assembler/live/EFI/fedora
/srv/tmp/override/usr-share-cosa/usr/share/coreos-assembler/live/EFI/fedora/grub.cfg
/srv/tmp/override/usr-share-cosa/usr/share/coreos-assembler/live/live
/srv/tmp/override/usr-share-cosa/usr/share/coreos-assembler/live/live/README-devel.md
/srv/tmp/override/usr-share-cosa/usr/share/coreos-assembler/live/live/zipl.prm
/srv/tmp/override/usr-share-cosa/usr/share/coreos-assembler/live/live/isolinux
/srv/tmp/override/usr-share-cosa/usr/share/coreos-assembler/live/live/isolinux/isolinux.cfg
/srv/tmp/override/usr-share-cosa/usr/share/coreos-assembler/live/live/isolinux/boot.msg
/srv/tmp/override/usr-share-cosa/usr/share/coreos-assembler/live/live/EFI
/srv/tmp/override/usr-share-cosa/usr/share/coreos-assembler/live/live/EFI/fedora
/srv/tmp/override/usr-share-cosa/usr/share/coreos-assembler/live/live/EFI/fedora/grub.cfg
```

This results in the same being duplicated in our images. Let's clean up
between runs so we can not have the duplicate folder/files.

Author: dustymabe

src/cmdlib.sh

@@ -400,7 +400,8 @@ prepare_compose_overlays() {
     fi
 
     if [ -d "${overridesdir}" ] || [ -d "${ovld}" ] || [ -d "${workdir}/src/yumrepos" ]; then
-        mkdir -p "${tmp_overridesdir}"
+        rm -rf "${tmp_overridesdir}"
+        mkdir  "${tmp_overridesdir}"
         cat > "${override_manifest}" <<EOF
 include: ${manifest}
 EOF