EmulatorPkg: Update DSC/FDF to use NetworkPkg's include fragment file.

EmulatorPkg: Update DSC/FDF to use NetworkPkg's include fragment file.

This patch updates the platform DSC/FDF files to use the include fragment
files provided by NetworkPkg.
The feature enabling flags in [Defines] section have been updated to use
the NetworkPkg's terms, and the value has been overridden with the original
default value on this platform.

Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Andrew Fish <afish@apple.com>
Cc: Ruiyu Ni <ruiyu.ni@intel.com>
Signed-off-by: Shenglei Zhang <shenglei.zhang@intel.com>
Reviewed-by: Ray Ni <ray.ni@intel.com>
Message-Id: <20190516081810.27840-5-shenglei.zhang@intel.com>
Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=1293
[lersek@redhat.com: add TianoCore Bugzilla reference]