kconfig: add help messages for --help (-h) and --silent (-s)

Add missing options and make the help message more readable.
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

scripts/kconfig/conf.c

@@ -496,9 +496,13 @@ static struct option long_opts[] = {
 
 static void conf_usage(const char *progname)
 {
-
-	printf("Usage: %s [-s] [option] <kconfig-file>\n", progname);
-	printf("[option] is _one_ of the following:\n");
+	printf("Usage: %s [options] <kconfig-file>\n", progname);
+	printf("\n");
+	printf("Generic options:\n");
+	printf("  -h, --help              Print this message and exit.\n");
+	printf("  -s, --silent            Do not print log.\n");
+	printf("\n");
+	printf("Mode options:\n");
 	printf("  --listnewconfig         List new options\n");
 	printf("  --helpnewconfig         List new options and help text\n");
 	printf("  --oldaskconfig          Start a new configuration using a line-oriented program\n");