ArmPlatformPkg/NorFlashDxe: initialize varstore headers eagerly

ArmPlatformPkg/NorFlashDxe: initialize varstore headers eagerly

The lazy initialization of the varstore FVB makes no longer sense at this
point:

- "mNorFlashInstanceTemplate.Initialize" is NULL;

- in NorFlashCreateInstance(), we only set Instance->Initialize to
  non-NULL -- namely NorFlashFvbInitialize() -- if the FVB stands for the
  variable store (see "ContainVariableStorage" / "SupportFvb");

- we call Instance->Initialize() from three places:

  - from NorFlashWriteSingleBlock(), which is too late for the variable
    read service ("variable write" depends on "variable read");

  - from InitializeFvAndVariableStoreHeaders(), but that is only reachable
    from NorFlashFvbInitialize(), i.e. recursively from
    Instance->Initialize() itself;

  - and from FvbRead(), which is never called by the variable driver, only
    by the FTW driver. However, the variable driver may read (not write)
    the memory-mapped varstore flash chip before the FTW driver is
    dispatched.

Therefore the lazy initialization is both superfluous and insufficient.
Initialize the varstore headers eagerly, before we install the FVB
protocol interface.

Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Leif Lindholm <leif.lindholm@linaro.org>
Cc: Steve Capper <steve.capper@linaro.org>
Cc: Supreeth Venkatesh <Supreeth.Venkatesh@arm.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Tested-by: Steve Capper <steve.capper@linaro.org>
Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>