Add DIB_IPA_COMPRESS_CMD option

This option allows to specifiy command for compress built initramfs image
for ironic-agent element. This command can be specified with arguments and
should read raw data from stdin and write compressed data to stdout.
Default if "gzip".

Change-Id: I0fdd2ab91d7bf7aaaa6cdd5278e3902d44c5b883

elements/ironic-agent/README.rst

@@ -18,6 +18,10 @@ Beyond installing the ironic-python-agent, this element does the following:
 * Install the certificate if any, which is set to the environment variable
   ``DIB_IPA_CERT`` for validating the authenticity by ironic-python-agent. The
   certificate can be self-signed certificate or CA certificate.
+* Compresses initramfs with command specified in environment variable
+  ``DIB_IPA_COMPRESS_CMD``, which is 'gzip' by default. This command should listen
+  for raw data from stdin and write compressed data to stdout. Command can be
+  with arguments.
 
 This element outputs three files:
 

elements/ironic-agent/cleanup.d/99-ramdisk-create

@@ -18,6 +18,8 @@ source $_LIB/img-functions
 IMAGE_PATH=$(readlink -f $IMAGE_NAME)
 cd $TARGET_ROOT
 
+DIB_IPA_COMPRESS_CMD="${DIB_IPA_COMPRESS_CMD:-gzip}"
+
 echo "#disabled" > ./tmp/fstab.new
 sudo mv ./tmp/fstab.new ./etc/fstab
 sudo ln -s ./sbin/init ./
@@ -42,7 +44,7 @@ sudo find . -xdev \
     -path './var/cache/*' -prune -o \
     -name '*.pyc' -prune -o \
     -name '*.pyo' -prune -o \
-    -print | sudo cpio -o -H newc | gzip > ${IMAGE_PATH}.initramfs
+    -print | sudo cpio -o -H newc | ${DIB_IPA_COMPRESS_CMD} > ${IMAGE_PATH}.initramfs
 
 select_boot_kernel_initrd $TARGET_ROOT
 sudo cp $BOOTDIR/$KERNEL ${IMAGE_PATH}.kernel