efi_loader: fix FinalEvents table if an EFI app invoked GetEventLog

efi_loader: fix FinalEvents table if an EFI app invoked GetEventLog

As described in the TCG spec [1] in sections 7.1.1 and 7.1.2 the FinalEvent
table should include events after GetEventLog has been called.  This
currently works for us as long as the kernel is the only EFI application
calling that.  Specifically we only implement what's described in 7.1.1.

So refactor the code a bit and support EFI application(s) calling
GetEventLog.  Events will now be logged in both the EventLog and FinalEvent
table as long as ExitBootServices haven't been invoked.

[1] https://trustedcomputinggroup.org/wp-content/uploads/EFI-Protocol-Specification-rev13-160330final.pdf

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