BeagleBoardPkg: create private PrePi implementation

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>