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: #295 (closed)
Signed-off-by: Paul Sokolovsky firstname.lastname@example.org