BeagleBoneBlackPkg: Enable PEI Dependencies in DSC
BeagleBoneBlackPkg: Restore SEC to transition to PEI

Restore the earlier removed SEC functionality to allow
NonTrustedWorldTransition ()
BeagleBoneBlackPkg: Add FIFO Clear Wait to SerialPortLib

Wait till FIFO clears before sending new character to SerialPortWrite().
Same for SerialPortRead().
BeagleBoneBlackPkg: Build single Fd for all phases

Build BEAGLEBONEBLACK_EFI.Fd consisting of all boot phase Fv files.
BeagleBoneBlackPkg: Build single FD for SEC and PEI

Generate single FD consisting of both SEC and PEI phases.
BeagleBoneBlackPkg: Load PEI from X-Loader

Skip SEC. Load PrePeiCoreEntryPoint from X-Loader.
BeagleBoneBlackPkg: Configure Stack Base Address

Set PcdCPUCoresStackBase according to AM335x Memory Map (TRM 26.1.3.2) for
CreatePpiList() to work. Use correct ArmPlatformLib.h.
BeagleBoneBlackPkg: Add ArmPlatformPkg/PrePeiCore

Add Pre-PEI Core that is called by the SPL on boot. SEC is no longer needed.
BeagleBoneBlackPkg: Use FixedPcdGet in PrePeiCore

Use FixedPcdGet32() and disable redundant calls.
BeagleBoneBlackPkg: Use BBB specific PrePeiCore

Change DSC, FDF to use BBB PrePeiCore.