efi_loader: Extend PCR's for firmware measurements

Firmwares before U-Boot may be capable of doing tpm measurements
and passing them to U-Boot in the form of eventlog. However there
may be scenarios where the firmwares don't have TPM driver and
are not capable of extending the measurements in the PCRs.
Based on TCG spec, if previous firnware has extended PCR's, PCR0
would not be 0. So, read the PCR0 to determine if the PCR's need
to be extended as eventlog is parsed or not.

Signed-off-by: Ruchika Gupta <ruchika.gupta@linaro.org>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Tested-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>