fix incorrect check for NO_WARN_RWX_SEGMENTS_SUPPORTED. Use "NOT DEFINED" instead of "NOT VAR" to properly detect whether the variable is unset.

Author: Tobi

hw/bsp/family_support.cmake

@@ -141,7 +141,7 @@ if (NOT FAMILY STREQUAL rp2040)
   endif()
 endif()
 
-if (NOT NO_WARN_RWX_SEGMENTS_SUPPORTED)
+if (NOT DEFINED NO_WARN_RWX_SEGMENTS_SUPPORTED)
   set(NO_WARN_RWX_SEGMENTS_SUPPORTED 1)
 endif()