u-boot: tpm: Enable Measured Boot

Measured Boot is the term used to describe the process of securely
recording and computing hashes of code and critical data at each stage
in the boot chain prior to their use.

These measurements can be employed by other system components to
establish a comprehensive attestation system. For example, they could be
employed to enforce local attestation policies (such as the release of
specific platform keys) or to securely transmit them to a remote
challenger, also known as a verifier, post-boot to verify the condition
of the code and critical data.

Measured launch does not authenticate the code or critical data; rather,
it records the code or critical data that was present on the system
during boot.

Initially, the TPM measures the BIOS/EFI layer in the fundamental flow.
This measurement involves the generation of a cryptographic hash of the
binary image and the verification of the binary instructions that this
layer will execute. The TPM stores the generated hash in one of the
numerous "slots" in the Platform Configuration Register (PCR). The TPM
or entities external to the TPM can read these portions of memory at a
later time; however, they are unalterable once they have been written.
These memory pieces are protected by integrity protection from the
instant they are first written. This guarantees that the value written
to a PCR by the TPM will remain constant for the duration of the system,
unless the system is powered off or rebooted.

Acked-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Signed-off-by: Javier Tia <javier.tia@linaro.org>
Signed-off-by: Armin Kuster <akuster808@gmail.com>

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