Add oflag=sync to the dd command line

[fwyzard]

Using only status=progress conv=fsync will result in a seemingly very fast copy, much faster than the write speed of the SD card, followed by a long delay as the data is actually written to the SD card.
Using status=progress conv=fsync oflag=sync will show a slower progress, roughly matching the write speed of the SD card, but once it is done the command will complete immediately.

I think the second approach is more intuitive and less likely to cause users to think the copy is hung.

[JamesH65]

Hmm, nice idea. Once I've confirmed it's all OK will merge.

[fwyzard]

In fact, I suspect conv=fsync can be dropped altogether if using oflag=sync,

And by the way, dd(1) suggests also oflag=direct, but I haven't tried it.

[lurch]

@fwyzard As you've noted, this is (apparently) only an issue when using the status=progress option in newer versions of dd. So perhaps this oflag=sync should only be used with the last dd command, and not with all three dd commands as this PR is currently doing?

(from what you're describing, it sounds like the conv=fsync does one big file-sync at the end, but the oflag=sync does lots of smaller file-syncs as the file is being written? Note that I've not tested any of these options myself)

[fwyzard]

OK.

Though it may be harder to explain why one uses different flags when showing the progress.

[aallan]

In light of #1911 and the lack of progress in a long while I'm marking this PR as 7 day closure.

[aallan]

No one picked this up inside the 7 day closure window. So closing due to #1911.