EXTRA_IMAGECMD:squashfs-xz support

["Bills, Jason M" <jason.m.bills@linux.intel.com>]

Hello,

I work on OpenBMC and am having an issue with EXTRA_IMAGECMD:squashfs-xz.

There was a recent change pulled into OpenBMC in image_types.bbclass. In 
the old file, IMAGE_CMD:squashfs-xz was defined directly:

IMAGE_CMD:squashfs-xz = "mksquashfs ${IMAGE_ROOTFS} 
${IMGDEPLOYDIR}/${IMAGE_NAME}${IMAGE_NAME_SUFFIX}.squashfs-xz 
${EXTRA_IMAGECMD} -noappend -comp xz"

In the new file, it is defined using a method:

oe_mksquashfs () {
     local comp=$1
     local suffix=$2

     # Use the bitbake reproducible timestamp instead of the hardcoded 
squashfs one
     export SOURCE_DATE_EPOCH=$(stat -c '%Y' ${IMAGE_ROOTFS})
     mksquashfs ${IMAGE_ROOTFS} 
${IMGDEPLOYDIR}/${IMAGE_NAME}${IMAGE_NAME_SUFFIX}.squashfs${comp:+-}${suffix:-$comp} 
${EXTRA_IMAGECMD} -noappend ${comp:+-comp }$comp
}
IMAGE_CMD:squashfs-xz = "oe_mksquashfs xz"

In my build we append to EXTRA_IMAGECMD:squashfs-xz to add parameters to 
the mksquashfs commmand like this:
EXTRA_IMAGECMD:squashfs-xz:append = "-processors ${BB_NUMBER_THREADS} -b 
262144 -Xdict-size 100% -Xbcj arm"

With the old file, the EXTRA_IMAGECMD:squashfs-xz parameters are added 
to the mksquashfs command. But with the new file, they don't get added 
anymore. I wonder if the method loses the squashfs-xz context so the 
empty EXTRA_IMAGECMD is used instead of EXTRA_IMAGECMD:squashfs-xz.

Should my EXTRA_IMAGECMD:squashfs-xz:append still work, or is there a 
different way to add parameters to the IMAGE_CMD:squashfs-xz command 
with the new approach?

Thanks!
-Jason