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.

build

@@ -76,7 +76,7 @@ BR2_TARGET_ROOTFS_INITRAMFS=n
 done
 shift $(($OPTIND - 1))
 extra_make_args="$extra_make_args $@"
-set_common_vars "$arch"
+set_common_vars "$arch" "$gem5"
 case "$arch" in
   x86_64)
     defconfig=qemu_x86_64_defconfig
@@ -91,9 +91,7 @@ case "$arch" in
     defconfig=qemu_mips64r6_malta_defconfig
   ;;
 esac
-if "$gem5"; then
-  post_script_args="$post_script_args -n"
-else
+if ! "$gem5"; then
   config_fragments="$config_fragments br2_qemu"
 fi
 config_fragments="$config_fragments br2_cli.gitignore"