EmbeddedPkg: Allow DXE_DRIVER to depend on NvVarStoreFormattedLib

EmbeddedPkg: Allow DXE_DRIVER to depend on NvVarStoreFormattedLib

Some modules (such as FaultTolerantWriteDxe) use the FlashNvStorage
PCDs (PcdFlashNvStorageFtw*). In case the flash contents are not
mapped in memory, the module loading order of the FVB driver
may become important.

To handle above, this patch allows to hook the dependency of
desired DXE_DRIVER type module in the .DSC file via
NvVarStoreFormattedLib NULL resolution.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Marcin Wojtas <mw@semihalf.com>
Reviewed-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>