ffboot: probe slots without a journal replay

A read-only mount of an ext4 slot still replays its journal, which
writes to the partition the probe was not meant to touch. The probe
mounts with noload: the journal is left as it is and the slot's
content is read as it stands.

Acceptance: update.slots covers ffboot -l (covers map ffboot/**).

meta-forgefirm/recipes-forgefirm/ffboot/files/ffboot

@@ -63,7 +63,7 @@ probe_part () {
     if [ -z "$ROOT_DIR" ]; then
       P_MOUNTED=yes
       ROOT_DIR=$(mktemp -d /tmp/ffboot.probe.XXXXXX) || return 1
-      if ! mount -o ro -t ext4 "$1" "$ROOT_DIR" 2>/dev/null; then
+      if ! mount -o ro,noload -t ext4 "$1" "$ROOT_DIR" 2>/dev/null; then
         rmdir "$ROOT_DIR" 2>/dev/null
         return 1
       fi