Create GENERIC_PROFILE_INDEX Macro (#196)

# Preface

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## Description

Add a macro to clarify that MAX_UINT32 is the generic profile index for
ActiveProfileIndexSelectorLib, instead of requiring consumers to know
this offhand.

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- [ ] Impacts functionality?
- **Functionality** - Does the change ultimately impact how firmware
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- **Security** - Does the change have a direct security impact on an
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    flow, or firmware?
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    validation improvement, ...
- [ ] Breaking change?
- **Breaking change** - Will anyone consuming this change experience a
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    in build or boot behavior?
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## How This Was Tested

Just a macro addition.

## Integration Instructions

N/A.

SetupDataPkg/Include/Library/ActiveProfileIndexSelectorLib.h

@@ -11,6 +11,9 @@
 #ifndef ACTIVE_PROFILE_INDEX_SELECTOR_LIB_H_
 #define ACTIVE_PROFILE_INDEX_SELECTOR_LIB_H_
 
+// macro to represent the generic profile, used as a default if no profile is selected or a failure occurs
+#define GENERIC_PROFILE_INDEX  MAX_UINT32
+
 /**
   Return which profile is the active profile for this boot.
   This function validates the profile GUID is valid.

SetupDataPkg/Library/ActiveProfileIndexSelectorLibNull/ActiveProfileIndexSelectorLibNull.c

@@ -9,6 +9,7 @@
 **/
 #include <Uefi.h>
 #include <Library/BaseLib.h>
+#include <Library/ActiveProfileIndexSelectorLib.h>
 
 /**
   Return which profile is the active profile for this boot.
@@ -34,7 +35,7 @@ GetActiveProfileIndex (
   }
 
   // Null instance, we just return the default profile
-  *ActiveProfileIndex = MAX_UINT32;
+  *ActiveProfileIndex = GENERIC_PROFILE_INDEX;
 
   return EFI_SUCCESS;
 }