You could get it bootable by copying it with
Code:
# cp -ax / /mnt/usbdisk
and installing the bootloader to the usbdisk.
Be aware that some computers only boot from usb zipdisk. So your 1st. partition should be of the size of a zip disk (100MB). Some computers require the disk to be formatted with vfat in order to be recognized.
Also be aware, that usb disks use flash memory which wear out with each write access to it. If you do not use tricks like using ramdisks for most of the system, you may easily trash your stick with daily work.
Better use special distributions for usb disks. Like Damn Small Linux or Puppy Linux.