Since dd prints everything on stderr, both normal status update and actual
errors when they occur, redirecting that to /dev/null is a trade-off that's
acceptable in quiet mode (typically used on a developer's machine and the
developer will re-run in non-quiet mode if anything fails without sufficient
detail in the output), but not that much in non-quiet mode.
For example, if our dd invocation fails because the disk in full on a CI
machine, we want the error to be reported at the time we invoke dd, and not
later when a seemingly unrelated test fails due to an incorrect seedfile.
Signed-off-by: Manuel Pégourié-Gonnard <manuel.pegourie-gonnard@arm.com>