dagger: Keep abandoned generations around for debugging

When evolving to a new generation fails, users will typically abandon
it (so that a new attempt can be made).  However, the generated
scripts and the logged output is very valuable from a debugging
perspective.

Therefore, keep the data around by just renaming the generation
directory.

Later, we can include these directories in the garbage collecting
process, if we want to.

board/common/rootfs/sbin/dagger

@@ -78,7 +78,8 @@ do_abandon()
     [ "$next" ] && [ -d "$basedir/$next" ] \
 	|| abort "Next generation does not exist"
 
-    rm -rf "$basedir/$next" "$basedir/next"
+    mv "$basedir/$next" "$basedir/$next-ABANDONED-$(date +%F-%T)"
+    rm "$basedir/next"
     inform info "Abandoned generation $next"
 }