cmake file(BASE_DIRECTORY) examples

YosysHQ/nextpnr cmake/BBAsm.cmake :15 

file(REAL_PATH ${EXPORT_BBA_FILES} EXPORT_BBA_FILES BASE_DIRECTORY ${CMAKE_BINARY_DIR})

YosysHQ/nextpnr cmake/BBAsm.cmake :20 

file(REAL_PATH ${IMPORT_BBA_FILES} IMPORT_BBA_FILES BASE_DIRECTORY ${CMAKE_BINARY_DIR})