Adds NixOS

src/linux.ipxe

@@ -21,6 +21,7 @@ item coreos ${space} CoreOS
 item devuan ${space} Devuan
 item gentoo ${space} Gentoo Linux
 item ipfire ${space} IPFire
+item nixos ${space} NixOS
 item rancheros ${space} RancherOS
 item rhel ${space} Red Hat Enterprise Linux
 item scientific ${space} Scientific

src/nixos.ipxe

@@ -0,0 +1,19 @@
+#!ipxe
+
+# NixOS
+# https://nixos.org/
+
+set os Nixos Linux
+menu ${os}
+item --gap Official Releases
+item 18.09 ${space} Nixos 18.09
+item unstable ${space} Nixos unstable
+choose version || goto nixos_exit
+iseq ${version} 18.09 && set link https://hydra.nixos.org/job/nixos/release-18.09/nixos.netboot.x86_64-linux/latest-finished/download/netboot.ipxe ||
+iseq ${version} unstable && set link https://hydra.nixos.org/job/nixos/trunk-combined/nixos.netboot.x86_64-linux/latest-finished/download/netboot.ipxe ||
+chain ${link}
+goto nixos_exit
+
+:nixos_exit
+clear menu
+exit 0