nodeagent: clarifies reboot reason message

SMART power cycle counter does not expose much and stays same
(previous counter and current counter) if system was reset,
rebooted or paniced. Previous quite pessimistic reboot message
said "kernel crash", which was not always the case and confused
our support and customers. More clear message should be
"system reset, reboot or kernel panic due to watchdog or kernel
bug (no kdump)".

Signed-off-by: Roman Penyaev <r.peniaev@gmail.com>

pkg/pillar/cmd/nodeagent/nodeagent.go

@@ -562,7 +562,7 @@ func handleLastRebootReason(ctx *nodeagentContext) {
 					dateStr)
 				bootReason = types.BootReasonPowerFail
 			} else {
-				reason = fmt.Sprintf("Reboot reason - kernel crash (no kdump) - at %s",
+				reason = fmt.Sprintf("Reboot reason - system reset, reboot or kernel panic due to watchdog or kernel bug (no kdump) - at %s",
 					dateStr)
 				bootReason = types.BootReasonKernel
 			}