imgtool: fix validation with protected TLVs

After the change to support protected TLVs, the `verify` command was not
updated with proper support. Fix it by skipping any protected TLV found,
and fix the size of the hashed/signed region to also include the
protected TLV area.

Signed-off-by: Fabio Utzig <fabio.utzig@nordicsemi.no>