bin/generate-zbm: require at least one kernel version value

Some distributions create generic symlinks to a kernel; such as vmlinuz.old.
These symlinks do not encode any version information in the filename. On ARM
systems, the kernel sometimes does not contain a version that can be detected
via dumping strings from the file. This leads to a case where neither version
string is defined, and the assumption was that one would always be defined.

bin/generate-zbm

@@ -609,6 +609,9 @@ sub kernelVersion {
     }
   }
 
+  # Kernel is unusable if no version could be detected
+  return if ( not nonempty $filever and not nonempty $namever );
+
   # If only one is defined, that's the version
   unless ( nonempty $filever ) {
     Log("No version found in kernel strings, using $namever from path $kernel");