Verilator DPI libraries when using MAIN

[Risto97]

Currently, if MAIN argument is used for Verilator backend function, DPI-C libraries are not supported.

Instead a standard flow must be used with executable and main defined explicitly like this:

ip_link(${IP} hello_dpi)
verilator(${IP})
add_executable(main Vtb__main.cpp)
target_link_libraries(main ${IP}__vlt)

The problem comes with the fact that the Vtb__main.cpp is generated during verilation.
Since SoCMake moves verilation from configure stage to build stage (through ExternalProject), the file is not available in the main CMake project.

There are few options:

1. Check how the Vtb_main.cpp is generated, if its simple templating we can generate it with configure_file() easily. Declare executable in the main CMake project.
2. Just create a placeholder main.cpp file, that will be overwritten in the external project. Then declare the executable in the main CMake project.
3. Keep executable in the external project and pass shared/static libraries as paths through -DLIBRARIES="<lib1.so>|<lib2.a>,..."