CryptoPkg/BaseCryptLib: replace HmacXxxInit API with HmacXxxSetKey

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=1792

HmacXxxInit() is supposed to be initialize user supplied buffer as HMAC
context, as well as user supplied key. Currently it has no real use cases.

Due to BZ1792, the user has no way to get correct size of context buffer
after it's fixed, and then cannot make use of HmacXxxInit to initialize
it.

So it's decided to replace it with HmacXxxSetKey to keep the functionality
of supplying a key to HMAC, but drop all other initialization works. The
user can still get HMAC context via HmacXxxNew interface, which hides the
details about the context.

Cc: Xiaoyu Lu <xiaoyux.lu@intel.com>
Cc: Laszlo Ersek <lersek@redhat.com>
Signed-off-by: Jian J Wang <jian.j.wang@intel.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Xiaoyu Lu <xiaoyux.lu@intel.com>

CryptoPkg/Library/BaseCryptLibNull/Hmac/CryptHmacMd5Null.c

@@ -65,12 +65,12 @@ HmacMd5Free (
 }
 
 /**
-  Initializes user-supplied memory pointed by HmacMd5Context as HMAC-MD5 context for
-  subsequent use.
+  Set user-supplied key for subsequent use. It must be done before any
+  calling to HmacMd5Update().
 
   Return FALSE to indicate this interface is not supported.
 
-  @param[out]  HmacMd5Context  Pointer to HMAC-MD5 context being initialized.
+  @param[out]  HmacMd5Context  Pointer to HMAC-MD5 context.
   @param[in]   Key             Pointer to the user-supplied key.
   @param[in]   KeySize         Key size in bytes.
 
@@ -79,7 +79,7 @@ HmacMd5Free (
 **/
 BOOLEAN
 EFIAPI
-HmacMd5Init (
+HmacMd5SetKey (
   OUT  VOID         *HmacMd5Context,
   IN   CONST UINT8  *Key,
   IN   UINTN        KeySize