cmake: fix setting defaults in idf_build_process

Closes https://github.com/espressif/esp-idf/issues/5053

tools/cmake/build.cmake

@@ -314,7 +314,7 @@ endmacro()
 #
 macro(__build_set_default var default)
     set(_var __${var})
-    if(NOT "${_var}" STREQUAL "")
+    if(NOT "${${_var}}" STREQUAL "")
         idf_build_set_property(${var} "${${_var}}")
     else()
         idf_build_set_property(${var} "${default}")