mbedtls: add PKCS7 parser porting layer

Add porting layer for PKCS7 parser on top of MbedTLS PKCS7 library.
Introduce _LEGACY and _MBEDTLS kconfigs for PKCS7 parser legacy and
MbedTLS implementations respectively.

Signed-off-by: Raymond Mao <raymond.mao@linaro.org>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
3 files changed