[Boardcore] Create library 'boardcore'

* Skyward Boardcore is now a library.

* CMake file 'boardcore.cmake' is added to define the library.

* Functions 'sbs_link_mxgui' and 'sbs_get_board' are no longer needed, as Skyward Build System targets are linked to Skyward Boardcore which in turn is linked to all libraries.