scripts: assemble: Fix problem with missing output
scripts: assemble: Fix problem with missing output

Adding an os.unlink() call to remove the outfile results in an exception
being thrown if the file does not exist.  Fix this by trapping, and
checking for the specific error we get on a missing file.

Signed-off-by: David Brown <0af22db655f3ade5845314d695c1f4910314b0cb@linaro.org>

ATmobica/mcuboot,runtimeco/mcuboot,utzig/mcuboot,utzig/mcuboot,ATmobica/mcuboot,tamban01/mcuboot,utzig/mcuboot,tamban01/mcuboot,runtimeco/mcuboot,ATmobica/mcuboot,utzig/mcuboot,tamban01/mcuboot,runtimeco/mcuboot,ATmobica/mcuboot,runtimeco/mcuboot,runtimeco/mcuboot,tamban01/mcuboot,utzig/mcuboot,tamban01/mcuboot,ATmobica/mcuboot