top-level Makefile improvements
top-level Makefile improvements

- auto-execute startup.vpp file if exists
- add DPDK_CONFIG=<string> to speficy custom config for autogenerated
  startup.conf, e.g. DPDK_CONFIG="no-pci"
- add DISABLED_PLUGINS=<comma separated list> option

e.g.

make DISABLED_PLUGINS=dpdk,acl run

or
export DISABLED_PLUGINS=dpdk,acl
make run

Change-Id: Ib0891d27aeae16b69b67c4c3e893bb3dbde5e7e1
Signed-off-by: Damjan Marion <9141bba8b2efed526e55cf796d48631af330ad98@cisco.com>

chrisy/vpp,chrisy/vpp,vpp-dev/vpp,FDio/vpp,FDio/vpp,FDio/vpp,chrisy/vpp,vpp-dev/vpp,FDio/vpp,chrisy/vpp,vpp-dev/vpp,chrisy/vpp,vpp-dev/vpp,FDio/vpp,FDio/vpp,vpp-dev/vpp,vpp-dev/vpp,vpp-dev/vpp,FDio/vpp,chrisy/vpp,FDio/vpp,chrisy/vpp,chrisy/vpp