board: imx: address dd usage in README

board: imx: address dd usage in README

When using cygwin64 "dd (coreutils) 8.26 Packaged by Cygwin (8.26-2)",
the last not 512bytes aligned data wat cut off and not burned into SD
card.

Saying the flash.bin size is 1085608 bytes, not 512bytes aligned. It only
burned 1085440 bytes, the leaving 168 bytes were not burnned and cause
boot issue.

So update README dd command to add "conv=notrunc"

Signed-off-by: Peng Fan <peng.fan@nxp.com>