Add Tss2_Tcti_Null_Context struct

[AndreasFuchsTPM]

tpm2-tss/include/tss2/tss2_tcti_null.h

Line 14 in 23d9c3c

Add

static const TSS2_TCTI_CONTEXT_COMMON_V2 Tss2_Tcti_Null_Context = {
    v1.magic = TCTI_NULL_MAGIC;
    v1.version = 2,
};

for quick access without requiring to call Tss2_Tcti_Null_Init() first...

[joholl]

I missed this completely. What do we use the null tcti for?

[JuergenReppSIT]

The idea was to enable the computation of the cp hash if no device exists. e.g.:
tpm2_getrandom -f 32 --cphash=cphash -Tnull

[joholl]

Ah thanks, that makes sense.

What came to my mind is incorporating the null tcti symbols in SAPI as weak symbols... (and giving a clear description of went wrong if the user forgets to link a real tcti and trying to communicating with the tpm). I did not think this through, though. I know I've struggled in the past with multiple definition of tcti symbols (tctildr, and tcti-fuzz etc.) so we should not do this until we understand all implications (i.e. for static linking etc.).