Hello,
There is work in progress by community members to implement PKCS#7 SignedData parsing and generation.
https://github.com/ARMmbed/mbedtls/pull/3970 https://github.com/ARMmbed/mbedtls/pull/3431
Arm has no particular plans in this area, but if you need other parts of PKCS#7, we'd be happy to accept more contributions. We'll can't commit to doing any development, but we'll assist with submissions and review code as usual.
mbed-tls@lists.trustedfirmware.org