Switch from gzip to lzma compression.

This signficantly increases time to generate Artifacts,
but also significantly reduces the Artifact sizes.

Example (gzip, lzma) on 2019-04-08 Raspbian conversion:
* Size: 354M -> 224M (-37%)
* Compression time: 1min -> 10min (10x)

Changelog: Use LZMA for smaller Artifact size (but slower generation).

Signed-off-by: Eystein Måløy Stenberg <eystein.maloy.stenberg@northern.tech>

device-image-shell/docker-entrypoint.sh

@@ -33,6 +33,6 @@ fi
 
 umount /root_system
 
-echo "Creating Mender Artifact. This may take a few minutes..."
-mender-artifact write rootfs-image -t $DEVICE_TYPE -n $ARTIFACT_NAME -f /root_images/$ROOTFS_OUTPUT_FILE_NAME -o /root_images/$ARTIFACT_NAME.mender
+echo "Creating Mender Artifact. This may take 10-20 minutes (using LZMA)..."
+mender-artifact --compression lzma write rootfs-image -t $DEVICE_TYPE -n $ARTIFACT_NAME -f /root_images/$ROOTFS_OUTPUT_FILE_NAME -o /root_images/$ARTIFACT_NAME.mender
 sync

mender-convert

@@ -615,9 +615,12 @@ do_mender_disk_image_to_artifact() {
       rm -rf ${output_dir}/mnt
 
       log "\tWriting Mender artifact to: ${mender_artifact}"
+      log "\tThis may take 10-20 minutes (using LZMA)..."
 
       #Create Mender artifact
-      mender-artifact write rootfs-image \
+      mender-artifact \
+        --compression lzma \
+        write rootfs-image \
         --file ${mender_rootfs_image} \
         --output-path ${mender_artifact} \
         --artifact-name ${artifact_name} \