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_post_build_script

@@ -5,19 +5,6 @@
 # To test it out, arguments can be passed with:
 #     make BR2_ROOTFS_POST_SCRIPT_ARGS="asdf qwer"
 target_dir="$1"
-shift
-net=true
-while getopts 'n' OPT; do
-  case "$OPT" in
-    n)
-      net=false
-    ;;
-  esac
-done
-shift $(($OPTIND - 1))
-if ! "$net"; then
-  rm -f "${target_dir}/etc/init.d/"S*network
-fi
 mkdir -p \
   "${target_dir}/mnt/9p" \
   "${target_dir}/mnt/out" \