MdeModulePkg/Bds: Do not boot to UI again when BootNext points to UI

Per UEFI spec the successful returning of boot option triggers boot
to UI. But when the BootNext just points to UI, it causes confusing.
So the patch avoids booting to UI again when the BootNext points to
UI.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Ruiyu Ni <ruiyu.ni@intel.com>
Reviewed-by: Eric Dong <eric.dong@intel.com>
Reviewed-by: Amy Chan <amy.chan@intel.com>