osbuild: supress disk usage log and add tips

The log disk usage message comming every 10 seconds is quite noisy,
hide it by default. We can always uncomment it to investigate.

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

Author: jbtrystram

src/cmd-osbuild

@@ -405,7 +405,12 @@ 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 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/runvm-osbuild

@@ -109,11 +109,16 @@ set -x; osbuild-mpp                                       \
     "${mppyaml}" "${processed_json}"
 set +x
 
-log_disk_usage
+#log_disk_usage
 
 # 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"          \