Add EmuVariableFvbRuntimeDxe driver.

Add EmuVariableFvbRuntimeDxe driver.

This driver implements a firmware volume block protocol instance
which is stored in system memory.

The MdeModulePkg/Universal/Variable/RuntimeDxe and
MdeModulePkg/Universal/FaultTolerantWriteDxe drivers make use of
this FVB instance to provide variable services.

This driver links to a PlatformFvb library to allow for
platform specific processing to take place when data is written
to the FVB.

git-svn-id: https://edk2.svn.sourceforge.net/svnroot/edk2/trunk/edk2@9315 6f19259b-4bc3-4df7-8a09-765794883524