ArmPlatformPkg/NorFlashDxe: cue the variable driver with NvVarStoreFormatted

The BEFORE depex opcode that we currently use to force ourselves in front
of the variable driver cannot be combined with other depex opcodes.
Replace the depex with TRUE, and signal NvVarStoreFormattedLib through the
installation of "gEdkiiNvVarStoreFormattedGuid".

Platforms that rely on NorFlashDxe to format the variable store (as
opposed to formatting a variable store template through an FDF file, as
part of the build) should hook NvVarStoreFormattedLib into the variable
drivers they use, so that the latter await our cue.

Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Leif Lindholm <leif.lindholm@linaro.org>
Cc: Steve Capper <steve.capper@linaro.org>
Cc: Supreeth Venkatesh <Supreeth.Venkatesh@arm.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Tested-by: Steve Capper <steve.capper@linaro.org>
Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>

ArmPlatformPkg/Drivers/NorFlashDxe/NorFlashFvbDxe.c

@@ -25,6 +25,7 @@
 
 #include <Guid/VariableFormat.h>
 #include <Guid/SystemNvDataGuid.h>
+#include <Guid/NvVarStoreFormatted.h>
 
 #include "NorFlashDxe.h"
 
@@ -776,6 +777,18 @@ NorFlashFvbInitialize (
     }
   }
 
+  //
+  // The driver implementing the variable read service can now be dispatched;
+  // the varstore headers are in place.
+  //
+  Status = gBS->InstallProtocolInterface (
+                  &gImageHandle,
+                  &gEdkiiNvVarStoreFormattedGuid,
+                  EFI_NATIVE_INTERFACE,
+                  NULL
+                  );
+  ASSERT_EFI_ERROR (Status);
+
   //
   // Register for the virtual address change event
   //