sdcard_image-rpi.bbclass: Allow optional compression

Image files will typically contain lots of null blocks and should compress well.
This will help with distribution of images over slow network links.

Change-Id: Icec7454ff61cd81a2872037037a93ce01191ece5
Signed-off-by: Paul Barker <paul@paulbarker.me.uk>

README

@@ -112,6 +112,8 @@ How to use it:
 	IMAGE_FSTYPES = "tar.bz2 ext3.xz rpi-sdimg.xz
 2. Overwrite SDIMG_ROOTFS_TYPE in local.conf
 	SDIMG_ROOTFS_TYPE = "ext3.xz"
+3. Overwrite SDIMG_COMPRESSION in local.conf
+	SDIMG_COMPRESSION = "xz"
 *Accommodate the values above to your own needs (ex: ext3 / ext4).
 
 2.B. Optional - GPU memory:

classes/sdcard_image-rpi.bbclass

@@ -50,6 +50,13 @@ IMAGE_DEPENDS_rpi-sdimg = " \
 # SD card image name
 SDIMG = "${DEPLOY_DIR_IMAGE}/${IMAGE_NAME}.rootfs.rpi-sdimg"
 
+# Compression method to apply to SDIMG after it has been created. Supported
+# compression formats are "gzip", "bzip2" or "xz". The original .rpi-sdimg file
+# is kept and a new compressed file is created if one of these compression
+# formats is chosen. If SDIMG_COMPRESSION is set to any other value it is
+# silently ignored.
+#SDIMG_COMPRESSION ?= ""
+
 # Additional files and/or directories to be copied into the vfat partition from the IMAGE_ROOTFS.
 FATPAYLOAD ?= ""
 
@@ -107,6 +114,19 @@ IMAGE_CMD_rpi-sdimg () {
 	else
 		dd if=${SDIMG_ROOTFS} of=${SDIMG} conv=notrunc seek=1 bs=$(expr 1024 \* ${BOOT_SPACE_ALIGNED} + ${IMAGE_ROOTFS_ALIGNMENT} \* 1024) && sync && sync
 	fi
+
+	# Optionally apply compression
+	case "${SDIMG_COMPRESSION}" in
+	"gzip")
+		gzip -k9 "${SDIMG}"
+		;;
+	"bzip2")
+		bzip2 -k9 "${SDIMG}"
+		;;
+	"xz")
+		xz -k "${SDIMG}"
+		;;
+	esac
 }
 
 ROOTFS_POSTPROCESS_COMMAND += " rpi_generate_sysctl_config ; "