units: add a tpm2.target synchronization point and small generator that pulls in

Distributions apparently only compile a subset of TPM2 drivers into the
kernel. For those not compiled it but provided as kmod we need a
synchronization point: we must wait before the first TPM2 interaction
until the driver is available and accessible.

This adds a tpm2.target unit as such a synchronization point. It's
ordered after /dev/tpmrm0, and is pulled in by a generator whenever we
detect that the kernel reported a TPM2 to exist but we have no device
for it yet.

This should solve the issue, but might create problems: if there are TPM
devices supported by firmware that we don't have Linux drivers for we'll
hang for a bit. Hence let's add a kernel cmdline switch to disable (or
alternatively force) this logic.

Fixes: #30164

src/basic/special.h

@@ -47,6 +47,7 @@
 #define SPECIAL_TIME_SYNC_TARGET "time-sync.target"       /* LSB's $time */
 #define SPECIAL_TIME_SET_TARGET "time-set.target"
 #define SPECIAL_BASIC_TARGET "basic.target"
+#define SPECIAL_TPM2_TARGET "tpm2.target"
 
 /* LSB compatibility */
 #define SPECIAL_NETWORK_TARGET "network.target"           /* LSB's $network */