Use losetup instead of kpartx to set up loopback partitions

kpartx on riscv64 appears to be racy.  Rather than trying to debug these
fraught races somewhere between udev and libdevmapper, we can use losetup
which should be simpler and less error-prone.

debian/control

@@ -22,11 +22,11 @@ Depends: ${misc:Depends},
          git,
          gnupg,
          grep-dctrl,
-         kpartx,
          live-build (>= 3.0~a57-1ubuntu31~),
          lsb-release,
          lzma,
          make,
+         mount,
          parted,
          procps,
          python3,

live-build/functions

@@ -12,16 +12,13 @@ loop_raw=
 backing_img=
 
 clean_loops() {
-    local kpartx_ret
-    local kpartx_stdout
-
     if [ -n "${backing_img}" ]; then
         # If something just finished writing to the device or a
         # partition (e.g. the zerofree in umount_partition) udev might
         # still be processing the device.
         udevadm settle
         sync
-        kpartx -v -d "${backing_img}"
+        losetup -v -d "${backing_img}"
         unset backing_img
     fi
 
@@ -65,18 +62,15 @@ mount_image() {
     trap clean_loops EXIT
     backing_img="$1"
     local rootpart="$2"
-    kpartx_mapping="$(kpartx -s -v -a ${backing_img})"
+    loop_device=$(losetup --show -f -P -v ${backing_img})
 
-    # Find the loop device
-    loop_p1="$(echo -e ${kpartx_mapping} | head -n1 | awk '{print$3}')"
-    loop_device="/dev/${loop_p1%p[0-9]*}"
     if [ ! -b ${loop_device} ]; then
         echo "unable to find loop device for ${backing_img}"
         exit 1
     fi
 
     # Find the rootfs location
-    rootfs_dev_mapper="/dev/mapper/${loop_p1%%[0-9]}${rootpart}"
+    rootfs_dev_mapper="/dev/mapper/${loop_device}p${rootpart}"
     if [ ! -b "${rootfs_dev_mapper}" ]; then
         echo "${rootfs_dev_mapper} is not a block device";
         exit 1