Add support for using PXE booting over HTTP

Uses lpxelinux.0 from syslinux 6.03 to load kernel and initrd.img over HTTP instead of TFTP. TFTP is only used to load lpxelinux.0.

dhcpd.conf instructs lpxelinux.0 to load boot information from http://IP/pxelinux.cfg/01-mac-address that is routed in /etc/nginx/sites-enabled/pxeboot rewrite rule to puavo-rest that generates the information. After that kernel and initrd.img are loaded from http://IP/image/vmlinuz and http://IP/image/initrd.img. The boot continues after that like before.