Reland "build_kernel_image: Use syslog format for serial consoles"

This is a reland of commit b867e091af8c6647ec48ecc5628618002d8951b1

Original change's description:
> build_kernel_image: Use syslog format for serial consoles
>
> RAM OOPS pstore is registered as a serial console, for which
> we don't prepend messages with log-level/facility by default
> (MSG_FORMAT_DEFAULT).  This makes crash reports inconsistent
> because sometimes we have <level> prefix and sometimes we
> don't.
>
> Switch to MSG_FORMAT_SYSLOG so that pstore messages will always
> have log-level/facility prefix.
>
> Example (after panic)
> =====================
>
> - base
>
> cat /sys/fs/pstore/console-ramoops-0
> [..]
> [   24.907552] input: kblda7219max HDMI/DP, pcm=5 Jack as /devices/platform/kbl_da7219_mx98357a/sound/card0/input10
> [   24.912106] input: kblda7219max HDMI/DP, pcm=6 Jack as /devices/platform/kbl_da7219_mx98357a/sound/card0/input11
> [   25.677432] init: send-boot-elog-metrics main process (3456) terminated with status 1
> [   26.466787] zram: Added device: zram0
> [   26.712950] device-mapper: verity: sha256 using implementation "sha256-avx2"
> [   26.833286] zram0: detected capacity change from 0 to 14289797120
> [   27.007454] Adding 13954876k swap on /dev/zram0.  Priority:-2 extents:1 across:13954876k SS
> [   29.009623] crash-reporter: Saved OS version
> [  120.544204] ------------[ cut here ]------------
> [  120.544231] kernel BUG at fs/drop_caches.c:54!
> [  120.544256] invalid opcode: 0000 [#1] PREEMPT SMP KASAN PTI
> [  120.544277] CPU: 1 PID: 4378 Comm: bash Tainted: G     U            5.4.290-kasan_outline #1
> [  120.544290] Hardware name: HP Nami/Nami, BIOS Google_Nami.10775.1.0 06/12/2018
> [  120.544316] RIP: 0010:drop_caches_sysctl_handler+0x9/0x10
> [..]
>
> - patched
>
> cat /sys/fs/pstore/console-ramoops-0
> [..]
> <12>[   26.029936] init: send-boot-elog-metrics main process (3472) terminated with status 1
> <6>[   26.953501] zram: Added device: zram0
> <6>[   27.129768] device-mapper: verity: sha256 using implementation "sha256-avx2"
> <6>[   27.504036] zram0: detected capacity change from 0 to 14289797120
> <6>[   27.635588] Adding 13954876k swap on /dev/zram0.  Priority:-2 extents:1 across:13954876k SS
> <12>[   29.553790] crash-reporter: Saved OS version
> <4>[   37.165960] ------------[ cut here ]------------
> <2>[   37.166000] kernel BUG at fs/drop_caches.c:54!
> <4>[   37.166029] invalid opcode: 0000 [#1] PREEMPT SMP KASAN PTI
> <4>[   37.166054] CPU: 3 PID: 3891 Comm: bash Tainted: G     U            5.4.290-kasan_outline #1
> <4>[   37.166070] Hardware name: HP Nami/Nami, BIOS Google_Nami.10775.1.0 06/12/2018
> <4>[   37.166100] RIP: 0010:drop_caches_sysctl_handler+0x9/0x10
> [..]
>
>
> BUG=b:414300955
> TEST=triggered panic() and examined /sys/fs/pstore/console-ramoops-0
>      before and after the patch
> TEST=tested on nami and strongbad
> Cq-Depend:chrome-internal:8274397
>
> Disallow-Recycled-Builds: test-failures
> Change-Id: I1724a092fee1a5e4fb3b2079dcfa2d68b2b05f87
> Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosutils/+/6549139
> Reviewed-by: Raul Rangel <rrangel@chromium.org>
> Commit-Queue: Sergey Senozhatsky <senozhatsky@chromium.org>
> Reviewed-by: Tomasz Figa <tfiga@chromium.org>
> Tested-by: Sergey Senozhatsky <senozhatsky@chromium.org>

Bug: b:414300955
Change-Id: I59fba5405964548dcc7e1b91499311299c737403
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosutils/+/6636950
Tested-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Commit-Queue: Sergey Senozhatsky <senozhatsky@chromium.org>
Reviewed-by: Jae Hoon Kim <kimjae@chromium.org>
Reviewed-by: Suleiman Souhlal <suleiman@google.com>
Reviewed-by: Raul Rangel <rrangel@chromium.org>