Platform/RaspberryPi: Enable Boot Discovery Policy.

Modify platform boot to check the value of BootDiscoveryPolicy variable
and use BootPolicyManager Protocol to connect devices specified by the
variable.

Signed-off-by: Grzegorz Bernacki <gjb@semihalf.com>
Reviewed-by: Sunny Wang <sunny.wang@arm.com>
Reviewed-by: Pete Batard <pete@akeo.ie>
Tested-by: Pete Batard <pete@akeo.ie>