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.

br2

@@ -28,6 +28,9 @@ BR2_TOOLCHAIN_BUILDROOT_CXX=y
 BR2_TOOLCHAIN_BUILDROOT_FORTRAN=y
 BR2_TOOLCHAIN_BUILDROOT_WCHAR=y
 
+# We use our own to turn it on and off at runtime.
+BR2_PACKAGE_IFUPDOWN_SCRIPTS=n
+
 # lscpu: TODO not installing?
 BR2_PACKAGE_UTIL_LINUX=y
 BR2_PACKAGE_UTIL_LINUX_BINARIES=y