Add timer to reboot RPC for NRF lighting example

[rgoliver]

Problem

Reboot RPC command doesn't return Status::OK. Instead it reboots immediately before returning, which can be difficult for the host code to handle.

Summary of Changes

Add a timer to reboot after the RPC returns, this ensures the host
code knows the RPC command was successfully received before the
device reboots.

Also reboot after doing a FDR.

Testing: Verified new RPC behavior worked as expected on lighting-app RPC build, with nRF52840

[woody-apple]

/rebase

[woody-apple]

Hi @rgoliver this needs a restyle, I tried to run it, but it appears things have diverged enough the merge won't work :(

[github-actions]

Size increase report for "nrfconnect-example-build" from 4e7505f

File	Section	File	VM
chip-lighting.elf	text	72	72
chip-lighting.elf	k_timer_area	56	56
chip-lighting.elf	device_handles	8	8

Full report output

BLOAT REPORT

Files found only in the build output:
    report.csv

Comparing ./master_artifact/chip-shell.elf and ./pull_artifact/chip-shell.elf:

sections,vmsize,filesize

Comparing ./master_artifact/chip-lock.elf and ./pull_artifact/chip-lock.elf:

sections,vmsize,filesize

Comparing ./master_artifact/chip-lighting.elf and ./pull_artifact/chip-lighting.elf:

sections,vmsize,filesize
.debug_info,0,307
.debug_loc,0,107
.strtab,0,101
.debug_abbrev,0,90
.debug_line,0,87
.symtab,0,80
text,72,72
k_timer_area,56,56
.debug_str,0,25
.debug_frame,0,16
.debug_aranges,0,8
device_handles,8,8
.shstrtab,0,-1
.debug_ranges,0,-40