merge_config.sh: use signal names compatible with dash and bash

The SIGHUP SIGINT and SIGTERM names caused failures when running
merge_config.sh with the dash shell.  Dropping the "SIG" component makes
the script work in both bash and dash.

Signed-off-by: Darren Hart <dvhart@linux.intel.com>
Acked-by: John Stultz <john.stultz@linaro.org>
Cc: Michal Marek <mmarek@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Michal Marek <mmarek@suse.cz>