samples: Add .gitignore to ignore output files
samples: Add .gitignore to ignore output files

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

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