cmake: do not check if sdkconfig exists

tools/cmake/project.cmake

@@ -331,10 +331,6 @@ macro(project project_name)
 
     if(SDKCONFIG)
         get_filename_component(sdkconfig "${SDKCONFIG}" ABSOLUTE)
-        if(NOT EXISTS "${sdkconfig}")
-            message(FATAL_ERROR "SDKCONFIG '${sdkconfig}' does not exist.")
-        endif()
-        set(sdkconfig ${SDKCONFIG})
     else()
         set(sdkconfig "${CMAKE_CURRENT_LIST_DIR}/sdkconfig")
     endif()