Breaks make_image_bootable out of build_image into a separate
script. In addition, it make a helper .sh in the OUTPUT_DIR
to let make_image_bootable be re-run on an image with the same
arguments that were passed in via build_image.
This change also removes the use of the boot/ directory in
OUTPUT_DIR.
TEST=build_image verified; image_to_usb --test_image; booted l13
build_image; image_to_vm; qemu booted
build_image verified; image_to_usb; booted l13
BUG=chromium-os:5081
Review URL: http://codereview.chromium.org/3066037
Change-Id: I627d678089aa71c353347f387ad5b14063fd4e7b
