dracut/ignition-setup: Use boot= karg if it exists

This will be used with bootc which will default to injecting
a karg for `boot`.

overlay.d/05core/usr/lib/dracut/modules.d/35coreos-ignition/coreos-ignition-setup-user.sh

@@ -14,6 +14,60 @@ copy_file_if_exists() {
 destination=/usr/lib/ignition
 mkdir -p $destination
 
+karg() {
+    local name="$1" value="${2:-}"
+    local cmdline=( $(</proc/cmdline) )
+    for arg in "${cmdline[@]}"; do
+        if [[ "${arg%%=*}" == "${name}" ]]; then
+            value="${arg#*=}"
+        fi
+    done
+    echo "${value}"
+}
+
+# Copied from
+# https://github.com/dracutdevs/dracut/blob/9491e599282d0d6bb12063eddbd192c0d2ce8acf/modules.d/99base/dracut-lib.sh#L586
+# rather than sourcing it.
+label_uuid_to_dev() {
+    local _dev
+    _dev="${1#block:}"
+    case "$_dev" in
+        LABEL=*)
+            echo "/dev/disk/by-label/$(echo "${_dev#LABEL=}" | sed 's,/,\\x2f,g;s, ,\\x20,g')"
+            ;;
+        PARTLABEL=*)
+            echo "/dev/disk/by-partlabel/$(echo "${_dev#PARTLABEL=}" | sed 's,/,\\x2f,g;s, ,\\x20,g')"
+            ;;
+        UUID=*)
+            echo "/dev/disk/by-uuid/$(echo "${_dev#UUID=}" | tr "[:upper:]" "[:lower:]")"
+            ;;
+        PARTUUID=*)
+            echo "/dev/disk/by-partuuid/$(echo "${_dev#PARTUUID=}" | tr "[:upper:]" "[:lower:]")"
+            ;;
+    esac
+}
+
+# This is copied from coreos-boot-mount-generator which we should likely run
+# in the initramfs too
+bootdev=/dev/disk/by-label/boot
+bootkarg=$(karg boot)
+mpath=$(karg rd.multipath)
+if [ -n "${mpath}" ] && [ "${mpath}" != 0 ]; then
+    bootdev=/dev/disk/by-label/dm-mpath-boot
+# Newer nodes inject boot=UUID=..., but we support a larger subset of the dracut/fips API
+elif [ -n "${bootkarg}" ]; then
+    # Adapted from https://github.com/dracutdevs/dracut/blob/9491e599282d0d6bb12063eddbd192c0d2ce8acf/modules.d/01fips/fips.sh#L17
+    case "$bootkarg" in
+        LABEL=* | UUID=* | PARTUUID=* | PARTLABEL=*)
+            bootdev="$(label_uuid_to_dev "$bootkarg")";;
+        /dev/*) bootdev=$bootkarg;;
+        *) echo "Unknown boot karg '${bootkarg}'; falling back to ${bootdev}";;
+    esac
+# This is used for the first boot only
+elif [ -f /run/coreos/bootfs_uuid ]; then
+    bootdev=/dev/disk/by-uuid/$(cat /run/coreos/bootfs_uuid)
+fi
+
 if is-live-image; then
     # Live image. If the user has supplied a config.ign via an appended
     # initrd, put it in the right place.
@@ -27,6 +81,6 @@ else
     mkdir -p $bootmnt
     # mount as read-only since we don't strictly need write access and we may be
     # running alongside other code that also has it mounted ro
-    mount -o ro /dev/disk/by-label/boot $bootmnt
+    mount -o ro $bootdev $bootmnt
     copy_file_if_exists "${bootmnt}/ignition/config.ign" "${destination}/user.ign"
 fi