lib: sha256: add feature sha256_hmac

Adds the support of the hmac based on sha256.
This implementation is based on rfc2104.

Signed-off-by: Philippe Reynes <philippe.reynes@softathome.com>
Reviewed-by: Raymond Mao <raymond.mao@linaro.org>
diff --git a/include/u-boot/sha256.h b/include/u-boot/sha256.h
index 44a9b52..99cf78e 100644
--- a/include/u-boot/sha256.h
+++ b/include/u-boot/sha256.h
@@ -45,4 +45,8 @@
 void sha256_csum_wd(const unsigned char *input, unsigned int ilen,
 		unsigned char *output, unsigned int chunk_sz);
 
+int sha256_hmac(const unsigned char *key, int keylen,
+		const unsigned char *input, unsigned int ilen,
+		unsigned char *output);
+
 #endif /* _SHA256_H */