MdeModulePkg Variable: Not get NV PCD in VariableWriteServiceInitialize
REF: https://bugzilla.tianocore.org/show_bug.cgi?id=1323
Merge EmuVariable and Real variable driver.
Add macro NV_STORAGE_VARIABLE_BASE.
Not get NV PCD in VariableWriteServiceInitialize, but in
FtwNotificationEvent/SmmFtwNotificationEvent, then
VariableWriteServiceInitialize could be not aware the NV
storage is real or emulated.
This patch prepares for adding emulated variable NV mode
support in VariableRuntimeDxe.