cmsis_dap: Explicitly perform filesystem sync before umount.

There's hardware/software combination when unmounting too soon can lead
to a failed device deployment (flashing). Despite teh fact that umount
should perform an underlying analog of sync, it doesn't seem when hitting
those peculiar combinations, while explicit sync resolves that issue.

For reference, the issues was seen with Ubuntu 18.04 on Asus Zenbook,
with docker-compose LAVA setup, talking to FRDM-K64F board.

Fixes: lava/lava#295

Signed-off-by: Paul Sokolovsky <paul.sokolovsky@linaro.org>
15 jobs for cmsis_dap-sync in 9 minutes and 12 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Test
passed #51723
amd64
dispatcher-debian-10

00:03:01

passed #51721
amd64
dispatcher-debian-9

00:03:21

passed #51724
amd64
server-debian-10

00:02:16

passed #51722
amd64
server-debian-9

00:02:39

 
  Analyze
passed #51726
amd64
black

00:00:34

passed #51728
amd64
code_quality

00:00:23

passed #51727
amd64
codestyle

00:00:37

passed #51730
amd64
job-schema

00:00:54

passed #51729
amd64
pylint

00:02:20

passed #51725
amd64-dind allowed to fail
sast

00:01:42

 
  Build
passed #51733
amd64
amd64/pkg-debian-10

00:01:18

passed #51732
amd64
amd64/pkg-debian-9

00:01:40

passed #51731
amd64
doc

00:01:15

 
  Docker
passed #51734
amd64-dind
amd64/lava-dispatcher

00:00:44

passed #51735
amd64-dind
amd64/lava-server

00:01:50