BeagleBoardPkg: create private PrePi implementation

Clone the PrePi implementation for BeagleBoardPkg, so we can start
removing some of the awkward optimizations that we'd rather not keep
in reference code. Note that we only clone the unicore ARM flavor,
which is all we need for BeagleBoard.

In the case of PrePi, it involves libraries included by the SEC
startup code that are exposed to DXE core via HOBs containing function
pointers, which forces us to keep the primary FV reserved in memory.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>

BeagleBoardPkg/PrePi/MainUniCore.c

@@ -0,0 +1,39 @@
+/** @file
+*
+*  Copyright (c) 2011, ARM Limited. All rights reserved.
+*  Copyright (c) 2017, Linaro, Ltd. All rights reserved.
+*
+*  This program and the accompanying materials
+*  are licensed and made available under the terms and conditions of the BSD License
+*  which accompanies this distribution.  The full text of the license may be found at
+*  http://opensource.org/licenses/bsd-license.php
+*
+*  THE PROGRAM IS DISTRIBUTED UNDER THE BSD LICENSE ON AN "AS IS" BASIS,
+*  WITHOUT WARRANTIES OR REPRESENTATIONS OF ANY KIND, EITHER EXPRESS OR IMPLIED.
+*
+**/
+
+#include "PrePi.h"
+
+VOID
+PrimaryMain (
+  IN  UINTN                     UefiMemoryBase,
+  IN  UINTN                     StacksBase,
+  IN  UINT64                    StartTimeStamp
+  )
+{
+  PrePiMain (UefiMemoryBase, StacksBase, StartTimeStamp);
+
+  // We must never return
+  ASSERT(FALSE);
+}
+
+VOID
+SecondaryMain (
+  IN  UINTN                     MpId
+  )
+{
+  // We must never get into this function on UniCore system
+  ASSERT(FALSE);
+}
+