sr: updater: Use custom bootloader msg offset for stage functions

Edify commands get_stage and set_stage need to be aware of the
custom bootloader msg offset because they write the stage directly
to the BCB.

Change-Id: I155d94ef0a875ddd896c5de0043b8039b9f23194
sr: updater: Fix multi-stage docs and code

* Fix reboot_now comments to reflect actual usage.

* Fix get_stage arg check, it only requires one arg (but allow two
  for compatibility).

Change-Id: I692d626185af24620382f7b318c2b77f643ccfb8
sr: Update to Patch Set 10