kconfig: silence a warning about undefined env variable

When using an environment variable to set a config entry,
don'twarn if it is not defined. Update doc accordingly.

docs/overview.txt

@@ -761,8 +761,8 @@ The kconfig language is a hacked version, vampirised from the Linux kernel
 The list of the most notable changes (at least the ones I remember) follows:
 - the CONFIG_ prefix has been replaced with CT_
 - a leading | in prompts is skipped, and subsequent leading spaces are not
-  trimmed
-- otherwise leading spaces are silently trimmed
+  trimmed; otherwise leading spaces are silently trimmed
+- removed the warning about undefined environment variable
 
 The kconfig parsers (conf and mconf) are not installed pre-built, but as
 source files. Thus you can have the directory where crosstool-NG is installed,

kconfig/symbol.c

@@ -976,6 +976,4 @@ void prop_add_env(const char *env)
 	p = getenv(env);
 	if (p)
 		sym_add_default(sym, p);
-	else
-		menu_warn(current_entry, "environment variable %s undefined", env);
 }