lib: Parallel emerge during ramfs/kernel builds

.. since it's possible that dependent packages can be emerge'ing for the
first time, then the non-parallel emerge'ing is not efficient.

BUG=none
TEST=./run_tests

Change-Id: I6cdccfe30084cedd8385681f01f25b706ac000f8
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/chromite/+/4229286
Reviewed-by: Mike Frysinger <vapier@chromium.org>
Tested-by: Jae Hoon Kim <kimjae@chromium.org>
Commit-Queue: Jae Hoon Kim <kimjae@chromium.org>
diff --git a/scripts/build_minios.py b/scripts/build_minios.py
index 67b048d..7140ba7 100644
--- a/scripts/build_minios.py
+++ b/scripts/build_minios.py
@@ -7,6 +7,7 @@
 And inserting them into the Chromium OS images.
 """
 
+import os
 import tempfile
 
 from chromite.lib import commandline
@@ -69,6 +70,13 @@
         "debug flags. Use with --mod-for-dev in case kernel is "
         "not already built or needs to be rebuilt.",
     )
+    parser.add_argument(
+        "--jobs",
+        type=int,
+        default=os.cpu_count(),
+        help="Number of packages to build in parallel. "
+        "(Default: %(default)s)",
+    )
     return parser
 
 
@@ -88,6 +96,7 @@
             opts.private_key,
             opts.keyblock,
             opts.serial,
+            opts.jobs,
             build_kernel,
             opts.mod_for_dev,
         )