merge_config.sh: Use the first file as the initial config

Take the first config fragment and use it verbatim as the initial config
set.  This avoids running the verification loop for the first file, as
nothing has actually been merged at this point.  This significantly
increases performance for large config fragments.

Signed-off-by: Josh Boyer <jwboyer@redhat.com>
Acked-by: John Stultz <john.stultz@linaro.org>
Acked-by: Darren Hart <dvhart@linux.intel.com>
Signed-off-by: Michal Marek <mmarek@suse.cz>