imgtool: add better image overrun checks

This breaks the check() routine into two, one to check the header, one
to check the trailer. The reason is that header checking must be
performed when loading the input binary, while trailer overrun check
must be done after the whole image (with TLVs) is built.

To support the option of saving encrypted TLVs during swap in the
bootloader, a new parameters was added to the create command, to
allow the user to provide a config that matches the bootloader build
option and to do proper image overrun checks.

Signed-off-by: Fabio Utzig <utzig@apache.org>