ArmPkg/PlatformBootManagerLib: call ESRT hooks at appropriate times

The ESRT management protocol needs to be invoked at the appropriate times
to get the ESRT config table to be published when the ReadyToBoot event
is signalled. So add this handling to the default ArmPkg implementation
of PlatformBootManagerLib.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>