mbedtls: enable support of hkdf

Adds the support of key derivation using
the scheme hkdf.

Reviewed-by: Raymond Mao <raymond.mao@linaro.org>
Signed-off-by: Philippe Reynes <philippe.reynes@softathome.com>
diff --git a/lib/mbedtls/Makefile b/lib/mbedtls/Makefile
index ce0a61e..e66c201 100644
--- a/lib/mbedtls/Makefile
+++ b/lib/mbedtls/Makefile
@@ -33,6 +33,8 @@
 	$(MBEDTLS_LIB_DIR)/sha256.o
 mbedtls_lib_crypto-$(CONFIG_$(SPL_)SHA512_MBEDTLS) += \
 	$(MBEDTLS_LIB_DIR)/sha512.o
+mbedtls_lib_crypto-$(CONFIG_$(SPL_)HKDF_MBEDTLS) += \
+	$(MBEDTLS_LIB_DIR)/hkdf.o
 
 # MbedTLS X509 library
 obj-$(CONFIG_MBEDTLS_LIB_X509) += mbedtls_lib_x509.o