add layer meta-netboot to enable network boot over NBD (Network Block Device)

To enable the build of network bootable images, the following line must be
added to conf/local.conf:

   INHERIT += "netboot"

This layer contains recipes for the following components:
* busybox: activate the built-in NBD client
* initramfs-netboot: contains the init script started by the kernel,
  responsible for mounting the remote root filesystem then pivoting and
  exec'ing systemd
* initramfs-netboot-image: image to specify for building the initrd

More details are available in meta-netboot/README.

Bug-AGL: SPEC-175

Change-Id: Id2328dd9233d238cde77311e64e58344be244988
Signed-off-by: Stephane Desneux <stephane.desneux@iot.bzh>