SecurityPkg/AuthVariableLib: fix GCC build error

Commit 53c6ff1803 ("SecurityPkg:AuthVariableLib:Implement ECR1707 for
Private Auth Variable", 2017-09-12) introduced the following build
failure under several GCC toolchain versions:

> SecurityPkg/Library/AuthVariableLib/AuthService.c: In function
> 'CalculatePrivAuthVarSignChainSHA256Digest':
> SecurityPkg/Library/AuthVariableLib/AuthService.c:1567:58: error:
> pointer targets in passing argument 3 of 'X509GetCommonName' differ in
> signedness [-Werror=pointer-sign]
>    Status = X509GetCommonName(SignerCert, SignerCertSize, CertCommonName, &CertCommonNameSize);
>                                                           ^~~~~~~~~~~~~~
> In file included from
> SecurityPkg/Library/AuthVariableLib/AuthServiceInternal.h:34:0,
>                  from
>                  SecurityPkg/Library/AuthVariableLib/AuthService.c:32:
> CryptoPkg/Include/Library/BaseCryptLib.h:2202:1: note: expected 'CHAR8 *
> {aka char *}' but argument is of type 'UINT8 * {aka unsigned char *}'
>  X509GetCommonName (
>  ^~~~~~~~~~~~~~~~~
> cc1: all warnings being treated as errors

Fix it by changing the type of "CertCommonName" to array-of-CHAR8.

Locations where "CertCommonName" is used in the
CalculatePrivAuthVarSignChainSHA256Digest() function:

- it is taken the size of -- not impacted by this patch;

- passed to X509GetCommonName() as an argument -- the patch fixes the
  build error;

- passed to Sha256Update() as argument for "IN CONST VOID  *Data" -- not
  impacted by the patch;

- passed to AsciiStrLen() as argument -- drop the now-superfluous explicit
  cast.

Since we are touching the Sha256Update() function call, fix the coding
style too:

- the line is overlong, so break each argument to its own line;

- insert a space between "AsciiStrLen" and the opening paren "(".

Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Chao Zhang <chao.b.zhang@intel.com>
Cc: Gary Lin <glin@suse.com>
Cc: Leif Lindholm <leif.lindholm@linaro.org>
Cc: Long Qin <qin.long@intel.com>
Reported-by: Gary Lin <glin@suse.com>
Suggested-by: Gary Lin <glin@suse.com>
Suggested-by: Long Qin <qin.long@intel.com>
Fixes: 53c6ff1803
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>

SecurityPkg/Library/AuthVariableLib/AuthService.c

@@ -1554,7 +1554,7 @@ CalculatePrivAuthVarSignChainSHA256Digest(
 {
   UINT8                   *TbsCert;
   UINTN                   TbsCertSize;
-  UINT8                   CertCommonName[128];
+  CHAR8                   CertCommonName[128];
   UINTN                   CertCommonNameSize;
   BOOLEAN                 CryptoStatus;
   EFI_STATUS              Status;
@@ -1590,7 +1590,11 @@ CalculatePrivAuthVarSignChainSHA256Digest(
   //
   // '\0' is forced in CertCommonName. No overflow issue
   //
-  CryptoStatus = Sha256Update (mHashCtx, CertCommonName, AsciiStrLen((CHAR8 *)CertCommonName));
+  CryptoStatus = Sha256Update (
+                   mHashCtx,
+                   CertCommonName,
+                   AsciiStrLen (CertCommonName)
+                   );
   if (!CryptoStatus) {
     return EFI_ABORTED;
   }