I forgot to mention that there is a work in progress to add PKCS#7 parsing: https://github.com/ARMmbed/mbedtls/pull/3431

This is an external contribution so its addition to Mbed TLS depends not only on us maintainers' review bandwidth, but also on the availability of the kind contributor.

I'm not familiar with .p7* formats so I don't know whether the support added by this pull request is sufficient to cover all of those.