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>

net/eth_bootdev.c

@@ -13,8 +13,8 @@
 #include <bootflow.h>
 #include <command.h>
 #include <bootmeth.h>
-#include <distro.h>
 #include <dm.h>
+#include <extlinux.h>
 #include <init.h>
 #include <log.h>
 #include <net.h>
@@ -36,7 +36,7 @@ static int eth_get_bootflow(struct udevice *dev, struct bootflow_iter *iter,
 		return log_msg_ret("check", ret);
 
 	/*
-	 * Like distro boot, this assumes there is only one Ethernet device.
+	 * Like extlinux boot, this assumes there is only one Ethernet device.
 	 * In this case, that means that @eth is ignored
 	 */