Disable networking at runtime with lkmc_nonet cli instead of build

This is necessary because qemu and gem5 now use the same build folder.

Separate ./run -e and -f for kernel options before and after the ' - '.
This was already the better thing to do when -E was introduced,
but lkmc_nonet prompted me to do it nicer now.

Use the common script to find the out_dir on every toplevel script.

Include usage man pages on README.

rootfs_overlay/etc/init.d/S40network

@@ -0,0 +1,44 @@
+#!/bin/sh
+
+# Copied from: package/ifupdown-scripts/S40network at
+# 8ce27bb9fee80a406a4199657ef90e3c315e7457
+#
+# We then just added this line to turn it off at runtime.
+#
+# We also considered copying the file at build to / and then
+# sourcing it from S98 to be a bit DRYer, but it wouldn't work
+# very well if some S50 requires networking to work.
+#
+# This became necessary nitially when aarch64 qemu and gem5
+# started using the same Buildroot output directory.
+[ -n "$lkmc_nonet" ] && exit 0
+
+#
+# Start the network....
+#
+
+# Debian ifupdown needs the /run/network lock directory
+mkdir -p /run/network
+
+case "$1" in
+  start)
+	printf "Starting network: "
+	/sbin/ifup -a
+	[ $? = 0 ] && echo "OK" || echo "FAIL"
+	;;
+  stop)
+	printf "Stopping network: "
+	/sbin/ifdown -a
+	[ $? = 0 ] && echo "OK" || echo "FAIL"
+	;;
+  restart|reload)
+	"$0" stop
+	"$0" start
+	;;
+  *)
+	echo "Usage: $0 {start|stop|restart}"
+	exit 1
+esac
+
+exit $?
+