merge_config.sh: Avoid creating unnessary source softlinks

Viresh noticed when using merge_config.sh that a source softlink
was being created even when he didn't specify the -O option.

The problem arises due to the previous commit 409f117e
which added the -O option. Basically if -O is not specified,
we still pass '-O=.' to the make command, which then generates
a source softlink to ./

This patch adds an extra check so if there is no -O specified
to merge_config.sh, we don't pass one on to make.

Cc: Zhangfei Gao <zhangfei.gao@linaro.org>
Cc: Bruce Ashfield <bruce.ashfield@windriver.com>
Acked-by: Darren Hart <dvhart@linux.intel.com>
Reported-and-tested-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: John Stultz <john.stultz@linaro.org>
Signed-off-by: Michal Marek <mmarek@suse.cz>