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.

common

@@ -1,6 +1,7 @@
 #!/usr/bin/env bash
 set_common_vars() {
   arch="$1"
+  gem5="$2"
   root_dir="$(pwd)"
   buildroot_dir="${root_dir}/buildroot"
   arch_dir="$arch"