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cmsis_dap: Wait CMSIS-DAP MSD remount cycle to confirm successful programming

After CMSIS-DAP MSD finished programming firmware (either successfully or
not), it "unmounts" and "remounts" the USB MSD device (actually, issues
MSD "media changed" notifications). We now wait for "unmount" and following
"remount", and check for failure file, FAIL.TXT. If it's present, we dump
it and properly fail the operation (which for example allows parent LAVA
boot-retry action to try it again).

Note the detection of MSD unmount/remount requires issues FS sync commands
(which we now do on Python level using os.sync(), instead of calling
external "sync" command). (Commenting os.sync() calls is a way to trigger
failure behavior on hardware which is susceptible to it.)

Finally, while this new behavior is made default, a "skip_autoremount_wait"
option is added to revert to the old behavior (of just calling sync once and
waiting static delay). It can be set in a device dict for boards which may
turn out to have issues with new process.

Signed-off-by: Paul Sokolovsky <paul.sokolovsky@linaro.org>