[shell]:Start CHIP event-loop by default during initialization.

[yufengwangca]

Problem

Currently, shell does not start event-loop by default, we need to first issue 'start' command to run device layer event loop before issue other device commands. And 'start' command is always needed.

Summary of Changes

Start CHIP event-loop by default during initialization.

[github-actions]

Size increase report for "nrfconnect-example-build" from 4bfade2

File	Section	File	VM
chip-shell.elf	text	60	60
chip-shell.elf	rodata	-8	-4
chip-shell.elf	log_const_sections	-12	-12

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
.debug_info,0,11424
.debug_str,0,3175
.debug_line,0,2038
.debug_abbrev,0,1822
.debug_loc,0,177
.strtab,0,111
.symtab,0,64
text,60,60
.debug_frame,0,36
.debug_aranges,0,32
.debug_ranges,0,16
.shstrtab,0,1
rodata,-4,-8
log_const_sections,-12,-12

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