Add more safeguards to image_to_usb.sh. List USB drives if --to is omitted

Add a few more safeguards to image_to_usb.sh so we don't accidentally overwrite a hard drive.  Also without the --to option, list USB drives

I've updated the CL to remove the space in the "] ;".  I'm not sure I understand your "size != 0" comment as if the device is just attached for charging, it won't go through the trouble of becoming a USB storage device.  Will chat in person.

[davidjames, 2010-05-27 22:04:18]

LGTM w/nits

http://codereview.chromium.org/2269003/diff/2001/3001
File src/platform/installer/chromeos-common.sh (right):

http://codereview.chromium.org/2269003/diff/2001/3001#newcode518
src/platform/installer/chromeos-common.sh:518: [ "$(cat ${sd}/removable)" = 1 ]
; then
1. Should we also check that the size is not equal to zero? Some devices just
use USB for charging, so you can't actually write anything to it.
2. In this file, it looks like the convention is to have no space between the
closing square bracket and the semicolon. This also applies below.

[Paul Stewart, 2010-05-27 22:52:37]

Fixed style.