Shorten "ppc64el" to "p64el" in the volume ID

The bullseye d-i RC1 build failed as the volume ID was too long

debian/changelog

@@ -13,6 +13,10 @@ debian-cd (3.1.34) UNRELEASED; urgency=medium
   [ Petter Reinholdtse ]
   * Include eatmydata deb for d-i to use offline. Closes: #986772
 
+  [ Steve McIntyre ]
+  * Shorten "ppc64el" to "p64el" in the volume ID. The bullseye d-i
+    RC1 build failed as the volume ID was too long.
+
  -- Wolfgang Schweer <wschweer@arcor.de>  Fri, 12 Feb 2021 22:07:27 +0100
 
 debian-cd (3.1.33) unstable; urgency=medium

tools/start_new_disc

@@ -69,6 +69,8 @@ if [ $NUM_ARCHES = 1 ] ; then
 		export DISKINFO="$DISKINFO_DISTRO ${DEBIAN_KERNEL} $DEBVERSION \"$CAPCODENAME\" - $OFFICIAL $ARCHES $DISKTYPE${FIRMWARE_TEXT} $BUILD_DATE"
 	    fi
             case "$ARCHES" in
+                ppc64el)
+                    export VOLID="$VOLID_BASE p64el $DISKNUMSTRING" ;;
                 powerpc)
                     export VOLID="$VOLID_BASE ppc $DISKNUMSTRING" ;;
                 mips64el)