U-Boot requires to access x509_internal.h, mbedtls_sha256_context and
mbedtls_sha1_context in the porting layer, and this requires to
enable MBEDTLS_ALLOW_PRIVATE_ACCESS.
Enable it to mscode and pkcs7_parser to fix a mbedtls internal building
error when X509 is selected.
Moreover, Move it to a separate file to avoid enabling it in multiple
places.
Signed-off-by: Raymond Mao <raymond.mao@linaro.org>
Acked-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
