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

ArmVirtPkg: 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.

v2:Add check for NETWORK_SNP_ENABLE and NETWORK_TLS_ENABLE.

Cc: Laszlo Ersek <lersek@redhat.com>
Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Julien Grall <julien.grall@linaro.org>
Signed-off-by: Shenglei Zhang <shenglei.zhang@intel.com>
Message-Id: <20190516081810.27840-4-shenglei.zhang@intel.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=1293
[lersek@redhat.com: add TianoCore Bugzilla reference]
[lersek@redhat.com: undo spurious whitespace changes]
[lersek@redhat.com: clarify SNP and TLS rejection messages]
Signed-off-by: Laszlo Ersek <lersek@redhat.com>