minios: Add CHROMEOS_VERSION_STRING to minios kernel.

Add version to minios kernel by putting it in as a kernel flag. On
the update engine side, the version can be read by outputting the
kernel configs with `dump_kernel_configs <MINIOS-A kernel>`.

BUG=b:187096368
TEST=minios_unittest.py
TEST=./build_image --board=zork-minios, unpack partitions,
call `dump_kernel_configs`. Verfiy version exists.

Cq-Depend: chromium:2979833
Change-Id: I529e6f37f2986496c7ccb3b901f29caca60a6101
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/chromite/+/2966949
Tested-by: Vyshu Khota <vyshu@chromium.org>
Commit-Queue: Vyshu Khota <vyshu@chromium.org>
Reviewed-by: Mike Frysinger <vapier@chromium.org>
Reviewed-by: Jae Hoon Kim <kimjae@chromium.org>
diff --git a/scripts/build_minios.py b/scripts/build_minios.py
index d0585ea..7742e83 100644
--- a/scripts/build_minios.py
+++ b/scripts/build_minios.py
@@ -19,6 +19,8 @@
   parser = commandline.ArgumentParser(description=__doc__)
   parser.add_argument('--board', '-b', '--build-target', required=True,
                       help='The board name.')
+  parser.add_argument('--version', required=True,
+                      help='The chromeos version string.')
   parser.add_argument('--image', type='path', required=True,
                       help='The path to the chromium os image.')
   parser.add_argument('--keys-dir', type='path',
@@ -46,7 +48,7 @@
   opts.Freeze()
 
   with tempfile.TemporaryDirectory() as work_dir:
-    kernel = minios.CreateMiniOsKernelImage(opts.board, work_dir,
+    kernel = minios.CreateMiniOsKernelImage(opts.board, opts.version, work_dir,
                                             opts.keys_dir, opts.public_key,
                                             opts.private_key, opts.keyblock,
                                             opts.serial)