Convert various app/util files to C++

[bzbarsky-apple]

Problem

We likely want to create a C++ API for #2168 and these files need to read from buffers.

Summary of Changes

Rename files from .c to .cpp and add extern "C" as needed to link.

[github-actions]

Size increase report for "esp32-example-build"

File	Section	File	VM
chip-wifi-echo.elf	.dram0.bss	0	-8
chip-wifi-echo.elf	.flash.text	-8	-8
chip-wifi-echo.elf	.flash.rodata	-40	-40

Full report output

BLOAT REPORT

Files found only in the build output:
    report.csv

Comparing ./master_artifact/chip-wifi-echo.elf and ./pull_artifact/chip-wifi-echo.elf:

sections,vmsize,filesize
.debug_info,0,21567
.debug_line,0,4660
.debug_abbrev,0,2317
.debug_str,0,256
.strtab,0,189
[Unmapped],0,40
.debug_ranges,0,24
.shstrtab,0,-1
.dram0.bss,-8,0
.flash.text,-8,-8
.symtab,0,-16
.flash.rodata,-40,-40
.debug_loc,0,-44

[github-actions]

Size increase report for "nrfconnect-example-build"

File	Section	File	VM
chip-lock.elf	shell_root_cmds_sections	4	4
chip-lock.elf	text	-20	-20
chip-lock.elf	rodata	-40	-40
chip-lighting.elf	text	-16	-16
chip-lighting.elf	rodata	-40	-40

Full report output

BLOAT REPORT

Files found only in the build output:
    report.csv

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

sections,vmsize,filesize
.debug_info,0,15833
.debug_line,0,4621
.debug_abbrev,0,1946
.debug_str,0,256
.strtab,0,179
shell_root_cmds_sections,4,4
.shstrtab,0,1
.debug_frame,0,-4
text,-20,-20
.symtab,0,-32
rodata,-40,-40
.debug_ranges,0,-80
.debug_loc,0,-724

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

sections,vmsize,filesize
.debug_info,0,15833
.debug_line,0,4621
.debug_abbrev,0,1946
.debug_str,0,256
.strtab,0,179
.shstrtab,0,1
.debug_frame,0,-4
text,-16,-16
.symtab,0,-32
rodata,-40,-40
.debug_ranges,0,-80
.debug_loc,0,-724

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

sections,vmsize,filesize