coreos-boot-edit: persist boot UUID in /run

The new `boot` karg will only show up on the second boot and onwards. So
for the first boot, let's persist it in `/run` to tell
`coreos-boot-mount-generator` the UUID to use.

overlay.d/05core/usr/lib/dracut/modules.d/35coreos-ignition/coreos-boot-edit.sh

@@ -45,4 +45,7 @@ if [ -z "${boot}" ]; then
         exit 1
     fi
     rdcore kargs --boot-mount ${bootmnt} --append boot=UUID=${UUID}
+    # but also put it in /run for the first boot real root mount
+    mkdir -p /run/coreos
+    echo "${UUID}" > /run/coreos/bootfs_uuid
 fi

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

@@ -102,6 +102,9 @@ elif [ -n "${bootkarg}" ]; then
         /dev/*) bootdev=$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
 
 # We mount read-only by default mostly to protect