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>
diff --git a/lib/mbedtls/Makefile b/lib/mbedtls/Makefile
index 2965332..128a29c 100644
--- a/lib/mbedtls/Makefile
+++ b/lib/mbedtls/Makefile
@@ -16,6 +16,7 @@
 	public_key.o
 obj-$(CONFIG_$(SPL_)X509_CERTIFICATE_PARSER_MBEDTLS) += \
 	x509_cert_parser.o
+obj-$(CONFIG_$(SPL_)PKCS7_MESSAGE_PARSER_MBEDTLS) += pkcs7_parser.o
 
 # MbedTLS crypto library
 obj-$(CONFIG_MBEDTLS_LIB) += mbedtls_lib_crypto.o
@@ -49,5 +50,5 @@
 mbedtls_lib_x509-$(CONFIG_$(SPL_)X509_CERTIFICATE_PARSER_MBEDTLS) += \
 	$(MBEDTLS_LIB_DIR)/x509_crl.o \
 	$(MBEDTLS_LIB_DIR)/x509_crt.o
-mbedtls_lib_x509-$(CONFIG_$(SPL_)PKCS7_MESSAGE_PARSER) += \
+mbedtls_lib_x509-$(CONFIG_$(SPL_)PKCS7_MESSAGE_PARSER_MBEDTLS) += \
 	$(MBEDTLS_LIB_DIR)/pkcs7.o