Changed zigbee sample to use signal-atomic

Changed zigbee sample to use signal-atomic

The signal atomic type is more reliable than bool, since it is
guaranteed to enforce thread safety. Also fixed the logic in the 'while'
loop so that this will function properly when the calls work.

Change-Id: I1ef4713f9e894592b1b25c874327bce810c83352
Signed-off-by: Joseph Morrow <joseph.l.morrow@intel.com>
Signed-off-by: Erich Keane <erich.keane@intel.com>
Reviewed-on: https://gerrit.iotivity.org/gerrit/2769
Tested-by: jenkins-iotivity <jenkins-iotivity@opendaylight.org>
Reviewed-by: Patrick Lankswert <patrick.lankswert@intel.com>