ArmPkg/PlatformBootManagerLib: call ProcessCapsules() only once

ARM platforms have no restriction on when a system firmware update
capsule can be applied, and so it is not necessary to call
ProcessCapsules() twice. So let's drop the first invocation that
occurs before EndOfDxe, and rewrite the second call so that all
capsule updates will be applied when the console is up and able to
provide progress feedback.

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