osbuild: make disk usage log conditional and add tips

The log disk usage message comming every 10 seconds is quite noisy,
hide it when we are in a shell in osbuild.

I aslo added a couple of helpful tips in comments given by @dustymabe
to work with osbuild.

Author: jbtrystram

src/cmd-osbuild

@@ -410,7 +410,12 @@ main() {
     else
         cmd="runvm_with_cache"
     fi
-    $cmd -- /usr/lib/coreos-assembler/runvm-osbuild                                                     \
+
+    # To get a shell in the osbuild supermin VM uncomment this.
+    # osbuild can then be started with `bash tmp/build.<artifact>/cmd.sh`
+    # See comment about checkpoints in runvm-osbuild
+    # RUNVM_SHELL=1 \
+    $cmd -- /usr/lib/coreos-assembler/runvm-osbuild                                         \
                 --config "${runvm_osbuild_config_json}"                                                 \
                 --mpp "/usr/lib/coreos-assembler/osbuild-manifests/coreos.osbuild.${basearch}.mpp.yaml" \
                 --outdir "${outdir}"                                                                    \

src/cmdlib.sh

@@ -774,7 +774,7 @@ rc=0
 if [ -z "${RUNVM_SHELL:-}" ]; then
   (cd ${workdir}; bash ${tmp_builddir}/cmd.sh |& tee /dev/virtio-ports/cosa-cmdout) || rc=\$?
 else
-  (cd ${workdir}; bash)
+  (cd ${workdir}; RUNVM_SHELL=${RUNVM_SHELL:-} bash)
 fi
 echo \$rc > ${rc_file}
 if [ -n "\${cachedev}" ]; then

src/runvm-osbuild

@@ -122,11 +122,18 @@ set -x; osbuild-mpp                                       \
     "${mppyaml}" "${processed_json}"
 set +x
 
-log_disk_usage
+if [[ -z "${RUNVM_SHELL:-}" ]]; then
+    log_disk_usage
+fi
 
 # Build the image
 set -x
 # shellcheck disable=SC2068
+# To stop osbuild at a given stage to inspect the state of
+# things you can add `--break stage_id` to the following.
+# eg : `--break org.osbuild.bootc.install-to-filesystem`
+# The osbuild environnement is set up under `/run/osbuild`
+# Use it in conjuction with `RUNVM_SHELL=1 in `cmd-osbuild`
 osbuild \
     --out "$outdir"              \
     --store "$storedir"          \