kconfig: require an exact match for "is not set" to disable CONFIG option

Currently, any string starting "is not set" disables a CONFIG option.

For example, "# CONFIG_FOO is not settled down" is accepted as valid
input, functioning the same as "# CONFIG_FOO is not set". It is a
long-standing oddity.

Check the line against the exact pattern "is not set".
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

scripts/kconfig/confdata.c

@@ -454,7 +454,7 @@ int conf_read_simple(const char *name, int def)
 			if (!p)
 				continue;
 			*p++ = 0;
-			if (strncmp(p, "is not set", 10))
+			if (strcmp(p, "is not set"))
 				continue;
 
 			val = "n";