They can all be made bootable
1. The PC has to support booting from a USB flash drive. There may be anywhere from 1-3 items to change in the BIOS to make this possible assuming your BIOS supports it. Some bios's may refer to your flash drive as a USB floppy or USB zip. Of course there are exceptions, perhaps some are covered here.
2. The flash drive must contain the boot/system files.
3. The flash drive must have bootsector area. This is done with special utilities.
4. References to "A:" drive lines in the autoexec.bat and/or config.sys files you copy to the drive after you make it bootable may result in errors.