board/mender/x86_64/post-image-efi.sh: fix bootstrap creation

As the mender_x86_64_efi_defconfig does not build the xz package, creating a
boostrap image with a lzma compression results in the following error on boot:

```
Error while handling bootstrap Artifact, continuing: invalid bootstrap
Artifact: readHeaderV3: handleHeaderReads: readHeader: readNext: Failed to get
next header: reader: error reading archive: unexpected EOF
```

This also results in the same error for the generated mender image, as it's
also created with lzma compression unconditionally.

Set the compression to none to resolve the above error.

Signed-off-by: Adam Duskett <adam.duskett@amarulasolutions.com>
Signed-off-by: Julien Olivain <ju.o@free.fr>

board/mender/x86_64/post-image-efi.sh

@@ -53,7 +53,7 @@ generate_mender_bootstrap_artifact() {
 
   "${HOST_DIR}"/bin/mender-artifact \
     write bootstrap-artifact \
-    --compression lzma \
+    --compression none \
     --artifact-name "${ARTIFACT_NAME}" \
     --device-type "${DEVICE_TYPE}" \
     --provides "rootfs-image.version:${ARTIFACT_NAME}" \
@@ -68,7 +68,7 @@ generate_mender_image() {
     echo "Creating ${BINARIES_DIR}/${DEVICE_TYPE}-${ARTIFACT_NAME}.mender"
     "${HOST_DIR}/bin/mender-artifact" \
         write rootfs-image \
-        --compression lzma \
+        --compression none \
         -t "${DEVICE_TYPE}" \
         -n "${BR2_VERSION}" \
         -f "${BINARIES_DIR}/rootfs.ext2" \