Platform/RaspberryPi/RPi3: Add platform boot manager and helper libs

The libraries decide and set the boot order between UEFI Shell, SD media
and/or USB media and allow for user configuration and override. Stale
boot options (non present media) are automatically removed on boot.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Pete Batard <pete@akeo.ie>
Reviewed-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Tested-by: Jeremy Linton <jeremy.linton@arm.com>