OvmfPkg: indicate enablement of flash variables with a dedicated PCD

OvmfPkg: indicate enablement of flash variables with a dedicated PCD

PcdFlashNvStorageVariableBase64 is used to arbitrate between
QemuFlashFvbServicesRuntimeDxe and EmuVariableFvbRuntimeDxe, but even the
latter driver sets it if we fall back to it.

Allow code running later than the startup of these drivers to know about
the availability of flash variables, through a dedicated PCD.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@14843 6f19259b-4bc3-4df7-8a09-765794883524