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In order to support multi-stage recovery packages, we add the
set_stage() and get_stage() functions, which store a short string
somewhere it can be accessed across invocations of recovery.  We also
add reboot_now() which updater can invoke to immediately reboot the
device, without doing normal recovery cleanup.  (It can also choose
whether to boot off the boot or recovery partition.)

If the stage string is of the form "#/#", recovery's UI will be
augmented with a simple indicator of what stage you're in, so it
doesn't look like a reboot loop.

Change-Id: I62f7ff0bc802b549c9bcf3cc154a6bad99f94603