sim: Add a checkpoint function to test for entries

sim: Add a checkpoint function to test for entries

When loading a checkpoint, it's sometimes desirable to be able to test
whether an entry within a secion exists. This is currently done
automatically in the UNSERIALIZE_OPT_SCALAR macro, but it isn't
possible to do for arrays, containers, or enums. Instead of adding
even more macros, add a helper function (CheckpointIn::entryExists())
that tests for the presence of an entry.

Change-Id: I4b4646b03276b889fd3916efefff3bd552317dbc
Signed-off-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>