boot: bootutil: add option to disable cryptographic check of the image

- If MCUBOOT_SIGN_RSA, MCUBOOT_SIGN_EC, MCUBOOT_SIGN_EC256 are not
  defined cryptographic signature check is skipped during validation.
  sha256 check is still retained. This reduces the binary size, at the
  expense of greatly reduced security. However this can be acceptable
  in some scenarios where cryptographic check is not required.

Signed-off-by: Arvin Farahmand <arvinf@ip-logix.com>