Adds efi support to hd-media

Current hd-media is legacy bios mode only.
This MR adds efi-64 support.

hd-media is the only boot image that can be mounted RW, which makes it nice for making derivatives; example: https://salsa.debian.org/debconf-video-team/ansible/blob/master/usbinst/mk_usb_installer.sh

Implementation:

• Move the legacy binaries into boot/syslinux, tell boot loader with: --directory /boot/syslinux/
• put efi binaries under EFI/BOOT - This is the default for EFI.
• EFI/BOOT/SYSLX64.CFG sets the path and uses the existing .cnf files.

Issues:

• syslinux efi does not display text the same as syslinux legacy. menus do not render the same.
• It seams sometimes it does not display at all.
• This makes it hard to test, and I am sure some crashes. Again, this is only with efi, so the things that do work makes this MR an improvement.

My use case replaces the .cnf with a single boot option that works fine. (see mk_usb_installer.sh above.)