Clean up cros_make_image_bootable and image_to_vm

Clean up cros_make_image_bootable and image_to_vm

image_to_vm.sh was running chromeos-setimage.  This script
was not meant to be run outside of a CrOS environment, but
its use here predates the cros_make_image_bootable script.

The change converts the rehashing and imaging over to using
cros_make_image_bootable.  The vm image uses the usb boot target but defaults
the usb drive to sda3.

update_bootloaders is fixed to not clobber existing syslinux
templates.

cros_make_image_bootable flag override support was broken and this fixes it.

TEST=build a new image wtih rootfs checking and without and ensure both woth with image_to_vm.  Also make sure they still boot normally on available hardware.
BUG=chromium-os:5939

[adlr, 2010-08-20 22:53:36]

http://codereview.chromium.org/3189018/diff/1/4
File update_bootloaders.sh (right):

http://codereview.chromium.org/3189018/diff/1/4#newcode158
update_bootloaders.sh:158: # Don't clobber if this is an update though.
why not clobber? shouldn't we always be giving an image the latest goodness?

[Will Drewry, 2010-08-21 00:24:39]

thanks!

http://codereview.chromium.org/3189018/diff/1/4
File update_bootloaders.sh (right):

http://codereview.chromium.org/3189018/diff/1/4#newcode158
update_bootloaders.sh:158: # Don't clobber if this is an update though.
On 2010/08/20 22:53:36, adlr wrote:
> why not clobber? shouldn't we always be giving an image the latest goodness?

Originally I was thinking it was so that a mod_image_* script could update the
ESP.  In reality, it should just patch the rootfs templates! I've removed this
part of the change.
Nice catch!

[adlr, 2010-08-21 00:27:51]

LGTM

[Will Drewry, 2010-08-21 00:30:26]

On 2010/08/21 00:24:39, Will Drewry wrote:
> thanks!
> 
> http://codereview.chromium.org/3189018/diff/1/4
> File update_bootloaders.sh (right):
> 
> http://codereview.chromium.org/3189018/diff/1/4#newcode158
> update_bootloaders.sh:158: # Don't clobber if this is an update though.
> On 2010/08/20 22:53:36, adlr wrote:
> > why not clobber? shouldn't we always be giving an image the latest goodness?
> 
> Originally I was thinking it was so that a mod_image_* script could update the
> ESP.  In reality, it should just patch the rootfs templates! I've removed this
> part of the change.
> Nice catch!

Hrm. Actually it means a mod_for_X couldn't change out the devices after the
hashes are computed, but I think that;s okay for now. It can just be a separate
change as it doesn't matter here.