tests: Use BusyBox compatible variant of dd, which doesn't support "oflag"
tests: Use BusyBox compatible variant of dd, which doesn't support "oflag"

Signed-off-by: Kristian Amlie <505e66ae45028a0596c853559221f0b72c1cee21@mender.io>

bboozzoo/meta-mender,bboozzoo/meta-mender,bboozzoo/meta-mender,bboozzoo/meta-mender