build: add option to warn on recursive dependency

This addes the option to treat recursive dependencies as warnings
instead of errors, by running make with WARN_RECURSIVE_DEP=1.

Note that the script/config targets will not get rebuilt when you add or
remove WARN_RECURSIVE_DEP while running make.  One must run
'make config-clean' before building config with a different setting.

Signed-off-by: Eneas U de Queiroz <cotequeiroz@gmail.com>

scripts/config/README

@@ -16,6 +16,10 @@ OpenWrt Buildroot:
  - reverted an upstream change that avoids writing symbols that are not
    visible to .config, which breaks OpenWrt busybox's '.config' generation
    logic.
+ - add a compilation option (-DWARN_RECURSIVE_DEP) to treat recursive deps
+   as a warning, avoiding a complete build failure because of unrelated or
+   minor recursive deps, or making a scrict check before commiting a change
+   that may cause one.
  - use pre-built *.lex.c *.tab.[ch] files by default, to avoid depending on
    flex & bison.  Rebuild/remove these files only if running make with
    BUILD_SHIPPED_FILES defined

scripts/config/symbol.c

@@ -1250,6 +1250,11 @@ struct symbol *sym_check_deps(struct symbol *sym)
 		sym->flags &= ~SYMBOL_CHECK;
 	}
 
+#ifdef WARN_RECURSIVE_DEP
+	if (sym2 && sym2 == sym)
+		sym2 = NULL;
+#endif
+
 	return sym2;
 }