[gn] add zephyr platform to gn build system

[e-rk]

Problem

Zephyr platform doesn't use GN.

Summary of Changes

This PR adds basic support for GN build system with Zephyr build environment.

fixes #2225

[e-rk]

I have removed this define deliberately.
I think it would be good to have this define set in CHIPDevicePlatformConfig.h, because we can use Kconfig defines directly there.

[turon]

I think this would be changing behavior on all CHIP platforms, not just nrfconnect. @mspang ?

[mspang]

It seems fine, that's why there's a change to src/platform/EFR32/CHIPDevicePlatformConfig.h in this PR.

[rwalker-apple]

@e-rk will you resolve conflicts, here?

[e-rk]

@rwalker-apple Done.

[github-actions]

Size increase report for "nrfconnect-example-build"

File	Section	File	VM
chip-nrf52840-lock-example.elf	bss	0	16
chip-nrf52840-lock-example.elf	[LOAD #1 [RWX]]	-4	-4
chip-nrf52840-lock-example.elf	datas	-8	-8
chip-nrf52840-lock-example.elf	[LOAD #3 [RW]]	0	-16
chip-nrf52840-lock-example.elf	init_array	-20	-20
chip-nrf52840-lock-example.elf	[LOAD #2 [RW]]	-24	-24
chip-nrf52840-lock-example.elf	rodata	-4812	-4812
chip-nrf52840-lock-example.elf	text	-31896	-31896

Full report output

BLOAT REPORT

Files found only in the build output:
    report.csv

Comparing ./master_artifact/chip-nrf52840-lock-example.elf and ./pull_artifact/chip-nrf52840-lock-example.elf:

sections,vmsize,filesize
.debug_loc,0,140327
.debug_info,0,80781
.debug_abbrev,0,12859
.debug_line,0,8586
.debug_ranges,0,6888
bss,16,0
[LOAD #1 [RWX]],-4,-4
[Unmapped],0,-5
datas,-8,-8
[LOAD #3 [RW]],-16,0
init_array,-20,-20
[LOAD #2 [RW]],-24,-24
rodata,-4812,-4812
.debug_aranges,0,-6856
.symtab,0,-15552
.debug_str,0,-21409
.strtab,0,-28827
text,-31896,-31896
.debug_frame,0,-38240

[github-actions]

Size increase report for "gn_nrf-example-build"

File	Section	File	VM

Full report output

BLOAT REPORT

Files found only in the build output:
    report.csv

[github-actions]

Size increase report for "gn_linux-example-build"

File	Section	File	VM

Full report output

BLOAT REPORT

Files found only in the build output:
    report.csv

[github-actions]

Size increase report for "gn_efr32-example-build"

File	Section	File	VM

Full report output

BLOAT REPORT

Files found only in the build output:
    report.csv

Comparing ./master_artifact/chip-efr32-lock-example.out and ./pull_artifact/chip-efr32-lock-example.out:

sections,vmsize,filesize

[github-actions]

Size increase report for "esp32-example-build"

File	Section	File	VM

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

[mspang]

@saurabhst @hawk248 @BroderickCarlin @jelderton ?