MBEDTLS_PKCS7_H macro

\file pkcs7.h PKCS #7 generic defines and structures https://tools.ietf.org/html/rfc2315 Note: For the time being, this implementation of the PKCS #7 cryptographic message syntax is a partial implementation of RFC 2315. Differences include: - The RFC specifies 6 different content types. The only type currently supported in Mbed TLS is the signed-data content type. - The only supported PKCS #7 Signed Data syntax version is version 1 - The RFC specifies support for BER. This implementation is limited to DER only. - The RFC specifies that multiple digest algorithms can be specified in the Signed Data type. Only one digest algorithm is supported in Mbed TLS. - The RFC specifies the Signed Data type can contain multiple X.509 or PKCS #6 extended certificates. In Mbed TLS, this list can only contain 0 or 1 certificates and they must be in X.509 format. - The RFC specifies the Signed Data type can contain certificate-revocation lists (CRLs). This implementation has no support for CRLs so it is assumed to be an empty list. - The RFC allows for SignerInfo structure to optionally contain unauthenticatedAttributes and authenticatedAttributes. In Mbed TLS it is assumed these fields are empty. - The RFC allows for the signed Data type to contain contentInfo. This implementation assumes the type is DATA and the content is empty.