Fix non-zero status exit on non secure boot system (sonic-net#2715)

What I did
Warm-reboot fails on kvm due to non-zero exit upon command
bootctl status 2>/dev/null | grep -c "Secure Boot: enabled"

How I did it
Added || true to return 0 when previous command fails.
Added CHECK_SECURE_UPGRADE_ENABLED to check output of previous command
Added debug logs

How to verify it
Run warm-reboot on kvm and physical device when increased verbosity. Expects debug log to indicate secure/non secure boot. Successful warm reboot

scripts/fast-reboot

@@ -614,11 +614,14 @@ if is_secureboot && grep -q aboot_machine= /host/machine.conf; then
     load_aboot_secureboot_kernel
 else
     # check if secure boot is enable in UEFI 
-    SECURE_UPGRADE_ENABLED=$(bootctl status 2>/dev/null | grep -c "Secure Boot: enabled")
-    if [ ${SECURE_UPGRADE_ENABLED} -eq 1 ]; then
-        load_kernel_secure
-    else
+    CHECK_SECURE_UPGRADE_ENABLED=0
+    SECURE_UPGRADE_ENABLED=$(bootctl status 2>/dev/null | grep -c "Secure Boot: enabled") || CHECK_SECURE_UPGRADE_ENABLED=$?
+    if [[ CHECK_SECURE_UPGRADE_ENABLED -ne 0 ]]; then
+        debug "Loading kernel without secure boot"
         load_kernel
+    else
+        debug "Loading kernel with secure boot"
+        load_kernel_secure
     fi
 fi