op-mode: T5514: Allow safe reboots to config defaults when config.boot is deleted
- Added flag to vyos.config_mgmt.unsaved_commits() that will tolerate missing config.boot for specific circumstances
- Shutdown/reboot uses this flag; config will regenerate from defaults after a reboot
(cherry picked from commit 8281383a09f12da20a1c9b4864b38ac3f541b48f)