merge_config.sh: Add option to specify output dir

Provide a -O option to specify dir to put generated .config
Then merge_config.sh does not need to be copied to target dir,
for easy re-usage in other script

Signed-off-by: Zhangfei Gao <zhangfei.gao@linaro.org>
Tested-by: Jon Medhurst (Tixy) <tixy@linaro.org>
Acked-by: John Stultz <john.stultz@linaro.org>
Signed-off-by: Michal Marek <mmarek@suse.cz>