Use p2p build script when user plugin does not implement both boxes

[108anup]

When the user plugin does not implement both the boxes, build.tcl still sources box build script from the user specified path. Ideally it should load the build script for the default p2p box if the user plugin does not implement a box.

Note, build.tcl does source the correct ${box_plugin}/${box}_address_map.v and ${box_plugin}/${box}_axi_crossbar.tcl, but does not source the correct build_${box}.tcl

Changes in this PR work with the command vivado -mode tcl -source ./build.tcl -tclargs -board_repo /datadrive/board-files -board au50 -impl 1 -post_impl 1 -user_plugin ../plugin/byte-counter and following repo structure:

├── plugin
│   ├── byte-counter
│   │   ├── box_250mhz
│   │   │   ├── box_250mhz_address_map_inst.vh
│   │   │   ├── box_250mhz_address_map.v
│   │   │   ├── box_250mhz_axi_crossbar.tcl
│   │   │   └── user_plugin_250mhz_inst.vh
│   │   ├── build_box_250mhz.tcl
│   │   └── byte_counter_250mhz.sv
│   └── p2p
│       ├── box_250mhz
│       │   ├── box_250mhz_address_map_inst.vh
│       │   ├── box_250mhz_address_map.v
│       │   ├── box_250mhz_axi_crossbar.tcl
│       │   └── user_plugin_250mhz_inst.vh
│       ├── box_322mhz
│       │   ├── box_322mhz_address_map_inst.vh
│       │   ├── box_322mhz_address_map.v
│       │   ├── box_322mhz_axi_crossbar.tcl
│       │   └── user_plugin_322mhz_inst.vh
│       ├── build_box_250mhz.tcl
│       ├── build_box_322mhz.tcl
│       ├── p2p_250mhz.sv
│       └── p2p_322mhz.sv