sim: Always compute protected size

Instead of having a running size that we try to always match the encoded
size of, compute it a bit more dynamically.  The size is still needed
before the contents can be filled in, so this doesn't gain too much, but
will make it easier to compute the rest of the TLV size based on the
actual encoding rather than lots of magic numbers.

Signed-off-by: David Brown <david.brown@linaro.org>