tpm2-util: add infra for allocating nvindex-based PCRs (aka "NvPCRs")

We'd like to measure various additional things into PCRs, but all
available ones to the OS are already used for various purposes. Hence,
let's introduce a new concept of "NV Index based PCRs", i.e. let's use
TPM2 nv indexes of type TPM2_NT_EXTEND that mostly behave like real
PCRs, but which we can allocate relatively freely from the nv index
space. Let's call these "fake" PCRs "NvPCRs".

My original intention was to get a fixed NV index range assigned from
the TCG, either for Linux or for systemd as a project, but this stalled
with no further updates from the TCG for more than a year and a half
now. I was told an NV index range to use though, even if it never was
officially assigned, hence this PR uses this by default. But the range
is configurable at build time, on purpose, so that downstreams have some
flexibility to change this if they want. To abstract the actual nvindex
number away we introduce a naming concept, so that nvindexes are
referenced by name string rather than number.

NvPCRs are defined in little JSON snippets in /usr/lib/nvpcr/*.nvpcr,
that match up index number and name, as well as pick a hash algorithm.

There's one complication: these nvindex (like any nvindex) can be
deleted by anyone with access to the TPM, and then be recreated. This
could be used to reset the NvPCRs to zero during runtime, which defeats
the whole point of them. Our way out: we measure a secret as first thing
after creation into the NvPCRs. (Or actually, we measure a per-NvPCR
secret we derive from a system secret via an HMAC of the NvPCR name) and
the nvindex handle). This "anchoring" secret is stored in /run/ +
/var/lib/ + ESP/XBOOTLDR (the latter encrypted as credential, locked to
the TPM), to make it available at the whole runtime of the OS.

meson.build

@@ -1343,6 +1343,7 @@ tpm2 = dependency('tss2-esys tss2-rc tss2-mu tss2-tcti-device',
 tpm2_cflags = tpm2.partial_dependency(includes: true, compile_args: true)
 conf.set10('HAVE_TPM2', tpm2.found())
 conf.set10('HAVE_TSS2_ESYS3', tpm2.found() and tpm2.version().version_compare('>= 3.0.0'))
+conf.set('TPM2_NVPCR_BASE', get_option('tpm2-nvpcr-base'))
 
 libdw = dependency('libdw',
                    required : get_option('elfutils'))
@@ -3028,6 +3029,7 @@ summary({
         'default user $PATH'                        : default_user_path != '' ? default_user_path : '(same as system services)',
         'systemd service watchdog'                  : service_watchdog == '' ? 'disabled' : service_watchdog,
         'time epoch'                                : f'@time_epoch@ (@alt_time_epoch@)',
+        'TPM2 nvpcr base'                           : run_command(sh, '-c', 'printf 0x%x @0@'.format(get_option('tpm2-nvpcr-base')), check : true).stdout()
 })
 
 # TODO: