rootfs: Add fetch-sshkeys systemd generator

Using a systemd generator to automatically enable
*fetch-sshkeys@.service* unit instances allows multiple users' keys to
be specified on the kernel command line, without explicitly enabling
the corresponding units.

rootfs/overlay/usr/lib/systemd/system-generators/fetch-sshkeys-generator

@@ -0,0 +1,25 @@
+#!/bin/sh
+
+enable_unit() {
+    mkdir -p "${normaldir}"/multi-user.target.wants
+    ln -s /lib/systemd/system/"$1" "${normaldir}"/multi-user.target.wants/
+}
+
+normaldir="$1"
+# shellcheck disable=SC2034
+earlydir="$2"
+# shellcheck disable=SC2034
+latedir="$3"
+
+# shellcheck disable=SC2046
+set -- $(cat /proc/cmdline)
+while [ $# -ge 1 ]; do
+    case "$1" in
+    sshkeys.*=*)
+        arg=${1#*.}
+        user=${arg%=*}
+        enable_unit fetch-sshkeys@"${user}".service
+        ;;
+    esac
+    shift
+done

rootfs/overlay/usr/lib/systemd/system/multi-user.target.wants/fetch-sshkeys@root.service

@@ -1 +0,0 @@
-../fetch-sshkeys@.service