avb: add support for named persistent values

AVB 2.0 spec. revision 1.1 introduces support for named persistent values
that must be tamper evident and allows AVB to store arbitrary key-value
pairs [1].

Introduce implementation of two additional AVB operations
read_persistent_value()/write_persistent_value() for retrieving/storing
named persistent values.

Correspondent pull request in the OP-TEE OS project repo [2].

[1]: https://android.googlesource.com/platform/external/avb/+/android-9.0.0_r22
[2]: https://github.com/OP-TEE/optee_os/pull/2699



Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Sam Protsenko <semen.protsenko@linaro.org>
Signed-off-by: Igor Opaniuk <igor.opaniuk@gmail.com>