Platform/RaspberryPi: add UEFI Shell to boot manager menu

Take advantage of a recent change to the core EDK2 BDS code that
makes boot options with the LOAD_OPTION_ACTIVE flag cleared visible
in the boot manager menu, so that they can be selected manually
while never being considered for automatic booting (unless selected
specifically via BootNext)

Signed-off-by: Ard Biesheuvel <ard.biesheuvel@arm.com>
Reviewed-by: Pete Batard <pete@akeo.ie>
Tested-by: Pete Batard <pete@akeo.ie>
Acked-by: Leif Lindholm <leif@nuviainc.com>