bootstd: Rename distro and syslinux to extlinux

We use the terms 'distro' to mean extlinux but they are not really the
same. 'Distro' could refer to any method of booting a distribution,
whereas extlinux is a particular method.

Also we sometimes use syslinux, but it is better to use the same term in
all cases.

Rename distro to syslinux and also update bootstd uses of syslinux to use
extlinux instead.

Signed-off-by: Simon Glass <sjg@chromium.org>

include/extlinux.h

@@ -0,0 +1,24 @@
+/* SPDX-License-Identifier: GPL-2.0+ */
+/*
+ * Copyright 2021 Google LLC
+ * Written by Simon Glass <sjg@chromium.org>
+ */
+
+#ifndef __extlinux_h
+#define __extlinux_h
+
+#define EXTLINUX_FNAME	"extlinux/extlinux.conf"
+
+/**
+ * struct extlinux_info - useful information for extlinux_getfile()
+ *
+ * @dev: bootmethod device being used to boot
+ * @bflow: bootflow being booted
+ */
+struct extlinux_info {
+	struct udevice *dev;
+	struct bootflow *bflow;
+	struct cmd_tbl *cmdtp;
+};
+
+#endif