kconfig: do not warn on missing env variable

In crosstool-NG, we use env variables for the backend mode.
So it is perfectly legit that these variables are not set.
So do not warn about it.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>

kconfig/symbol.c

@@ -1255,6 +1255,4 @@ static 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);
 }