credentials: add policy that can allow key=null creds from the ESP (#42555)

This PR only sets the default to "relaxed" - I can change the default
to "tofu" if desired. But for that we will also need to update the NEWS
file to ensure everyone is aware of this new default.

---

This PR adds a new `systemd.credentials-boot=` 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 `relaxed` which is exactly the behavior we had before.

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

---

With that we can drop `- firstboot: optionally accept credentials at
firstboot without authentication` from TODO.md

docs/CREDENTIALS.md

@@ -274,6 +274,33 @@ SetCredentialEncrypted=foobar: \
 …
 ```
 
+### Null-Key Encryption and the Boot Policy
+
+In some situations a credential must be provisioned to a machine that does not
+yet (or does not at all) possess a key to encrypt against, for example before
+first boot completes, or on systems without a TPM2 device. For these cases
+`systemd-creds encrypt --with-key=null` produces a credential wrapped in the
+normal `systemd-creds` envelope, but encrypted with a fixed zero-length key.
+Such a credential travels through the same path as any other encrypted
+credential (e.g. it may be dropped into the ESP under `/loader/credentials/` or
+`*.efi.extra.d/`), but it offers neither confidentiality nor authenticity.
+Because of that, accepting it is risky on a system that could do better, and
+whether it is accepted at boot is controlled by the `systemd.credentials_boot_policy=`
+kernel command line option:
+
+| Mode      | A null-key credential is accepted when…           |
+|-----------|---------------------------------------------------|
+| `strict`  | never                                             |
+| `tofu`    | this is the first boot, or no TPM2 is available   |
+| `relaxed` | SecureBoot is off, or no TPM2 is available        |
+| `off`     | always                                            |
+
+The default is `relaxed`. This policy only governs the
+default acceptance decision; credentials encrypted against a host key or TPM2
+device are always accepted. See
+[systemd(1)](https://www.freedesktop.org/software/systemd/man/latest/systemd.html)
+for details.
+
 ## Inheritance from Container Managers, Hypervisors, Kernel Command Line, or the UEFI Boot Environment
 
 Sometimes it is useful to parameterize whole systems the same way as services,