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.

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,