TCG PC Client PFP spec requires to measure the secure
boot policy before validating the UEFI image.
This commit adds the secure boot variable measurement
of "SecureBoot", "PK", "KEK", "db", "dbx", "dbt", and "dbr".

Note that this implementation assumes that secure boot
variables are pre-configured and not be set/updated in runtime.
Signed-off-by: NMasahisa Kojima <masahisa.kojima@linaro.org>