Consider zero a valid kernel version.

SAFT testing requires changing kernel version to one level
below the current value (set to 1). This change allows
version number set to zero for test purposes.

Change-Id: Ia6f11578d9a6bc8c5544c56413c5589011d6334a

BUG=chromium-os:1976
TEST=manual

Ran `vbutil_kernel --repack --version 0 <other params>'
it used to fail, now it succeeds. This is also verified by
using in http://codereview.chromium.org/3781016 to support
TPM testing.

Review URL: http://codereview.chromium.org/3968006
diff --git a/utility/vbutil_kernel.c b/utility/vbutil_kernel.c
index 23b6eb2..cb47681 100644
--- a/utility/vbutil_kernel.c
+++ b/utility/vbutil_kernel.c
@@ -750,7 +750,7 @@
   char* key_block_file = NULL;
   char* signpubkey = NULL;
   char* signprivate = NULL;
-  uint64_t version = 0;
+  int version = -1;
   char* vmlinuz = NULL;
   char* bootloader = NULL;
   char* config_file = NULL;
@@ -862,7 +862,7 @@
       return r;
 
     case OPT_MODE_REPACK:
-      if (!config_file && !key_block_file && !version) {
+      if (!config_file && !key_block_file && (version<0)) {
         fprintf(stderr,
                 "You must supply at least one of "
                 "--config, --keyblock or --version\n");
@@ -874,8 +874,8 @@
         return 1;
       r = ReplaceConfig(bp, config_file);
       if (!r) {
-        if (version) {
-          bp->kernel_version = version;
+        if (version >= 0) {
+		bp->kernel_version = (uint64_t) version;
         }
         r = Pack(filename, key_block_file, signprivate, bp, pad, vblockonly);
       }