Clarify last-reboot-time is updated during a power-on event

[dplore]

Fixes #1171

[OpenConfigBot]

No major YANG version changes in commit 3edd9fd

[s19nal]

As discussed at the OC operators meeting, the simplest change to clarify the description defining what this leaf is intended to represent is the least disruptive. Pedantically the leaf name last-reboot-time remains slightly misleading, but that could always be another PR if anyone feels strongly enough about it?

[dplore]

@rgwilton @earies @LimeHat @nandanarista for any comments

[LimeHat]

looks good to me.

[rolandphung]

With last-poweroff-reason and this semantic change to last-reboot-time to be defined to be similar to boot-time, should we consider deprecating last-reboot-reason?

[AnandJyrm]

@dplore Wouldn't it be better to add a new leaf called boot-time/component-boot-time indicating the boot time of the component? openconfig-system boot-time could indicate the boot-time of active controller card in a modular system or chassis in case of a fixed system.

[earies]

Few generic comments/questions

• Should verbiage/expectations be added around reboots surrounding image upgrades, ZTP re-instantiation, etc.. where such values may not be retained?
• For POSIX compatible systems, see influence/structure from utmpx - e.g. last command, /var/log/wtmp - are distinct leafs needed vs. a flexible structure, historical events, etc..?