Enable use of the isohybrid tool

The isohybrid tool (included with syslinux) allows users to create a bootable USB image from the CD images we distribute. However, in order for this to work the CD images must include isolinux.bin, not isolinux-debug.bin. Otherwise you will get an error like this:

$ isohybrid adelie-full-x86_64-1.0-alpha4-20171223.iso 
isohybrid: adelie-full-x86_64-1.0-alpha4-20171223.iso: boot loader does not have an isolinux.bin
hybrid signature. Note that isolinux-debug.bin does not support hybrid booting

This change is made for pmmx and x86_64.

I then used this change and built a new CD image, then ran isohybrid adelie-full-x86_64-.... The resulting image (it has the same filename by default) can be pushed to a USB drive like so (taken from syslinux's documentation):

# dd bs=2048 if=/path/to/adelie.iso of=/dev/sdX status=progress

Arch uses bs=4M oflag=sync. Someone smarter than me can determine if this is needed.

No further changes were necessary to boot from USB at that point, other than fiddling with boot order and such.

Something else to consider is whether Adélie should distribute these hybrid images pre-made, or if it should be left to the user.