VyOS bootfiles for PXE

Normally, this folder should contain three files, required for booting via PXE:

Getting files

If you only need to install VyOS via PXE, you may get these files from an ISO image.
If you want to load VyOS with a configuration via PXE (for example, to use without installation), you may get all three files, using Ansible playbook with the next settings:
```
ansible-playbook qemu.yml -e iso_local=/tmp/vyos/custom_image.iso -e cloud_init=true -e cloud_init_ds=NoCloud,None -e pxe=true
```
Then, an archive with files will be located in the /tmp/ directory. Optionally, you may add the -e keep_user=true option to keep the vyos user for debugging purposes.