ArmPkg: Enable boot discovery policy for ARM package.

ArmPkg: Enable boot discovery policy for ARM package.

This commit adds code which check BootDiscoveryPolicy variable and
calls Boot Policy Manager Protocol to connect device specified by
the variable. To enable that mechanism for platform
EfiMdeModulePkgTokenSpaceGuid.PcdBootDiscoveryPolicy PCD must be
added to DSC file and BootDiscoveryPolicyUiLib should be added to
UiApp libraries.

Signed-off-by: Grzegorz Bernacki <gjb@semihalf.com>
Reviewed-by: Sunny Wang <sunny.wang@arm.com>
Reviewed-by: Sami Mujawar <sami.mujawar@arm.com>