Select factory image when --factory is given

Select factory image when --factory is given

prepare_test_image would prepare the correct image (test or factory)
but its return value would always specify the test image. This meant
that image_to_usb.sh would pick up the wrong image.

This commit fixes this.

BUG=chromium-os:12377
TEST=./image_to_usb --from=... --to=/dev/sdb
also with --factory
also with --test

Make sure it picks up the correct image in each case.

Change-Id: I165b604b448a6c938a47e57a667bd5f001626efe

Committed: http://chrome-svn/viewvc/chromeos?view=rev&revision=6e448ae

[sjg, 2011-03-03 05:26:35]

Nick I'm not sure what happened here, but clearly it wasn't using the right
image in the factory case.

image_to_vm is subtly different. Chris said that it doesn't have the problem. I
can't immediately see why not, but willing to believe it. In any case this CL
should have the same effect on image_to_vm.

[sosa, 2011-03-03 18:29:48]

LGTM