crypto: hmac - add hmac IPAD/OPAD constant

Many HMAC users directly use directly 0x36/0x5c values.
It's better with crypto to use a name instead of directly some crypto
constant.

This patch simply add HMAC_IPAD_VALUE/HMAC_OPAD_VALUE defines in a new
include file "crypto/hmac.h" and use them in crypto/hmac.c

Signed-off-by: Corentin Labbe <clabbe.montjoie@gmail.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

include/crypto/hmac.h

@@ -0,0 +1,7 @@
+#ifndef _CRYPTO_HMAC_H
+#define _CRYPTO_HMAC_H
+
+#define HMAC_IPAD_VALUE 0x36
+#define HMAC_OPAD_VALUE 0x5c
+
+#endif /* _CRYPTO_HMAC_H */