vpp: add VPP IPv4 forward use case

1. Add VPP in build.sh
2. Add VPP IPv4 forward use case to quickstart guide and user guide

Signed-off-by: Tianyu Li <tianyu.li@arm.com>
Reviewed-by: Honnappa Nagarahalli <honnappa.nagarahalli@arm.com>
Reviewed-by: Lijian Zhang <Lijian.Zhang@arm.com>
Reviewed-by: Nathan Brown <Nathan.Brown@arm.com>

Change-Id: I88a48c818ed6f7fe70bc94776ef87cbe739466a9