MdeModulePkg/BdsDxe: Avoid overwriting PlatformRecovery####

MdeModulePkg/BdsDxe: Avoid overwriting PlatformRecovery####

Current implementation always creates PlatformRecovery0000
pointing to \EFI\BOOT\BOOT$(ARCH).efi but it may overwrite
PlatformRecovery#### created before (maybe by a DXE driver).

The patch only uses the smallest unused option number for
the \EFI\BOOT\BOOT$(ARCH).efi PlatformRecovery#### to avoid
overwriting already-created PlatformRecovery####.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Ruiyu Ni <ruiyu.ni@intel.com>
Reviewed-by: Jie Lin <jie.lin@intel.com>
Reviewed-by: Sunny Wang <sunnywang@hpe.com>