efi_loader: Add an S-CRTM even for firmware version

TCG PC Client Platform Firmware Profile Spec mandates that an S-CRTM
event for the version identifier using the event type EV_S_CRTM_VERSION
must be measured.

So since we are trying to add more conformance into U-Boot, let's add
the event using U_BOOT_VERSION_STRING, extend PCR[0] accordingly and log
it in the EventLog

Signed-off-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>