OvmfPkg: use DxeTpmMeasurementLib if and only if TPM2_ENABLE

OvmfPkg: use DxeTpmMeasurementLib if and only if TPM2_ENABLE

(a) OvmfPkg first had to resolve the TpmMeasurementLib class -- for
    SECURE_BOOT_ENABLE only -- when the DxeImageVerificationLib instance
    became dependent on TpmMeasurementLib. For details, refer to commit
    0d28d286bf4d ("OvmfPkg: resolve TpmMeasurementLib dependency
    introduced in r14687", 2013-09-21).

(b) At the time, only one instance of TpmMeasurementLib existed, namely
    DxeTpmMeasurementLib. This lib instance didn't do anything -- like it
    was desirable for OVMF --, because OVMF didn't include any Tcg / TrEE
    protocol implementations.

(c) In commit 308521b13354 ("MdeModulePkg: Move TpmMeasurementLib
    LibraryClass from SecurityPkg", 2015-07-01), TpmMeasurementLibNull was
    introduced.

(d) In commit 285542ebbb03 ("OvmfPkg: Link AuthVariableLib for following
    merged variable driver deploy", 2015-07-01), a TpmMeasurementLib
    resolution became necessary regardless of SECURE_BOOT_ENABLE. And so
    TpmMeasurementLib was resolved to TpmMeasurementLibNull in OVMF, but
    only in the non-SECURE_BOOT_ENABLE case. This step -- possibly, the
    larger series containing commit 285542ebbb03 -- missed an opportunity
    for simplification: given (b), the DxeTpmMeasurementLib instance
    should have been simply replaced with the TpmMeasurementLibNull
    instance, regardless of SECURE_BOOT_ENABLE.

(e) In commit 1abfa4ce4835 ("Add TPM2 support defined in trusted computing
    group.", 2015-08-13), the TrEE dependency was replaced with a Tcg2
    dependency in DxeTpmMeasurementLib.

(f) Starting with commit 0c0a50d6b3ff ("OvmfPkg: include Tcg2Dxe module",
    2018-03-09), OVMF would include a Tcg2 protocol implementation,
    thereby satisfying DxeTpmMeasurementLib's dependency. With
    TPM2_ENABLE, it would actually make sense to consume
    DxeTpmMeasurementLib -- however, DxeTpmMeasurementLib would never be
    used without SECURE_BOOT_ENABLE.

Therefore, we have the following four scenarios:

- TPM2_ENABLE + SECURE_BOOT_ENABLE: works as expected.

- Neither enabled: works as expected.

- Only TPM2_ENABLE: this build is currently incorrect, because
  Variable/RuntimeDxe consumes TpmMeasurementLib directly, but
  TpmMeasureAndLogData() will never reach the TPM because we link
  TpmMeasurementLibNull into the variable driver. This is a problem from
  the larger series containing (f).

- Only SECURE_BOOT_ENABLE: this build works as expected, but it is
  wasteful -- given that the protocol database will never contain Tcg2
  without TPM2_ENABLE, we should simply use TpmMeasurementLibNull. This is
  a problem from (d).

Resolving TpmMeasurementLib to DxeTpmMeasurementLib as a function of
*only* TPM2_ENABLE, we can fix / optimize the last two cases.

v2:
  - Amend the title and description suggested by Laszlo
  - Move TpmMeasurementLib to the existed TPM2_ENABLE block

Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Laszlo Ersek <lersek@redhat.com>
Cc: Marc-André Lureau <marcandre.lureau@redhat.com>
Cc: Stefan Berger <stefanb@linux.ibm.com>
Signed-off-by: Gary Lin <glin@suse.com>
Message-Id: <20190704040731.5303-1-glin@suse.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>