zip-img: create, zips all QEMU images

build-all: make gem5 rebuild optional

rootfs_post_image_script: prevent qemu-img from generating trace files

rootfs_post_image_script

@@ -4,5 +4,7 @@ cd "$images_dir"
 f=rootfs.ext2
 exe="${HOST_DIR}/bin/qemu-img"
 if [ -f "$exe" ] && [ -f "$f" ]; then
-   "$exe" convert -f raw -O qcow2 "$f" "${f}.qcow2"
+  # TODO why does qemu-img produce traces?
+  # http://lists.nongnu.org/archive/html/qemu-discuss/2018-04/msg00019.html
+  "$exe" -T "pr_manager_run,file=/dev/null" convert -f raw -O qcow2 "$f" "${f}.qcow2"
 fi