coreos-boot-mount-generator: support boot= karg

This is analogous to `systemd-fstab-generator` parsing `root=` kargs.
There is a precedent for this in the FIPS module. In fact, we lift code
from there to make sure we're API compatible.

The goal of supporting a `boot` karg is to eliminate possible races for
the `by-label/boot` symlink in the real root if multiple exist.

Part of: coreos/fedora-coreos-tracker#976

overlay.d/05core/usr/lib/systemd/system-generators/coreos-boot-mount-generator

@@ -61,15 +61,47 @@ EOF
     add_wants "${unit_name}"
 }
 
+# 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
+}
+
 # If the root device is multipath, hook up /boot to use that too,
 # based on our custom udev rules in 90-coreos-device-mapper.rules
 # that creates "label found on mpath" links.
 # Otherwise, use the usual by-label symlink.
 # See discussion in https://github.com/coreos/fedora-coreos-config/pull/1022
 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
 fi
 
 # We mount read-only by default mostly to protect