credentials: add policy that can allow key=null creds from the ESP

This commit adds a new `systemd.credentials_boot_policy=` kernel
commandline that allows to control if credentials with
a `null` key are accepted.

The possible options are:
* strict: always insist on tpm encryption
* tofu: allow null encryption in firstboot mode and when no tpm is available
* relaxed: allow null encryption when sb is off, or no tpm is available
* off: allow null encryption always

The default is currently `relaxed` which is the same behavior
as before.

This replaces the initial idea of using plaintext credentials
at firstboot (thanks to Lennart for this nicer and simpler design).

Note that this also moves `in_first_boot()` to `basic/initrd-util`
which is a better fit now.

man/systemd.xml

@@ -965,6 +965,41 @@
         <xi:include href="version-info.xml" xpointer="v251"/></listitem>
       </varlistentry>
 
+      <varlistentry>
+        <term><varname>systemd.credentials_boot_policy=</varname></term>
+
+        <listitem><para>Controls under which conditions a credential encrypted with a <literal>null</literal>
+        key (i.e. a credential wrapped in a <command>systemd-creds</command> envelope but offering neither
+        confidentiality nor authenticity, see
+        <citerefentry><refentrytitle>systemd-creds</refentrytitle><manvolnum>1</manvolnum></citerefentry>)
+        is accepted at boot. This provides a fallback for provisioning credentials before a machine has a key
+        to encrypt them against, for example during first boot or on systems without a TPM2 device. Takes
+        one of <option>strict</option>, <option>tofu</option>, <option>relaxed</option> or
+        <option>off</option>:</para>
+
+        <itemizedlist>
+          <listitem><para><option>strict</option> never accepts a <literal>null</literal> key, i.e. always
+          insists on proper encryption.</para></listitem>
+
+          <listitem><para><option>tofu</option> (trust on first use) accepts a <literal>null</literal> key
+          during first boot, or when no TPM2 device is available.</para></listitem>
+
+          <listitem><para><option>relaxed</option> accepts a <literal>null</literal> key when SecureBoot is
+          disabled, or when no TPM2 device is available.</para></listitem>
+
+          <listitem><para><option>off</option> always accepts a <literal>null</literal> key.</para></listitem>
+        </itemizedlist>
+
+        <para>Defaults to <option>relaxed</option>. Note that this policy only applies to the default acceptance
+        decision; it has no effect on credentials encrypted against a host key or TPM2 device, which are
+        always accepted.</para>
+
+        <para>For further information see <ulink url="https://systemd.io/CREDENTIALS">System and Service
+        Credentials</ulink> documentation.</para>
+
+        <xi:include href="version-info.xml" xpointer="v262"/></listitem>
+      </varlistentry>
+
       <varlistentry>
         <term><varname>quiet</varname></term>