Switch Flatcar to https

Closes: #1712

Author: antonym

roles/netbootxyz/templates/menu/flatcar.ipxe.j2

@@ -30,7 +30,7 @@ goto flatcar_exit
 :alpha
 :edge
 set release ${menu}
-set base-url http://${release}.release.flatcar-linux.net/${os_arch}-usr/current
+set base-url https://${release}.release.flatcar-linux.net/${os_arch}-usr/current
 kernel ${base-url}/flatcar_production_pxe.vmlinuz ${flatcar_firstboot} ${flatcar_params} flatcar.autologin=tty1 flatcar.autologin=ttyS0 initrd=flatcar_production_pxe_image.cpio.gz ${cmdline}
 initrd ${base-url}/flatcar_production_pxe_image.cpio.gz
 boot