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.

src/basic/initrd-util.h

@@ -5,3 +5,5 @@
 
 bool in_initrd(void);
 void in_initrd_force(bool value);
+
+bool in_first_boot(void);