This is a revert of [0], which added some problematic short circuit
logic to prepare_ramdisk. This patch proposes a better fix that was
suggested in the reviews of [0], which is to generate the boot script
on start up rather than during prepare_ramdisk. The iPXE boot script
is not dependent on node properties, or anything that is determined
during deployment, so generating the boot script on start up should
not harm any current functionality.
[0] 2c18f22f99
Change-Id: Id545d6cf93227cf1fc2ff0c05dbdceb8fb6aa5b9
bc60c21d63