chromite: Extract minios paygen

Create a flag that when passed, extracts the miniOS partition
instead of the root and kernel partitions.

BUG=b:188193014
TEST=cros_generate_update_payload
--src-image zork-minios/latest/chromiumos_test_image.bin
--tgt-image zork-minios/R92-13946.0.2021_05_13_1545-a1/chromiumos_test_image.bin
--output mini.bin --minios

TEST=cros_generate_update_payload
--tgt-image zork-minios/R92-13946.0.2021_05_13_1545-a1/chromiumos_test_image.bin
--output mini.bin --minios --check

Change-Id: I42d914a301656f1740c7e784299067d67f921ea7
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/chromite/+/2904067
Tested-by: Vyshu Khota <vyshu@chromium.org>
Reviewed-by: Amin Hassani <ahassani@chromium.org>
Reviewed-by: George Engelbrecht <engeg@google.com>
Reviewed-by: Jae Hoon Kim <kimjae@chromium.org>
Commit-Queue: Vyshu Khota <vyshu@chromium.org>
diff --git a/scripts/cros_generate_update_payload_unittest.py b/scripts/cros_generate_update_payload_unittest.py
index 80f1247..8aa65fe 100644
--- a/scripts/cros_generate_update_payload_unittest.py
+++ b/scripts/cros_generate_update_payload_unittest.py
@@ -25,7 +25,7 @@
         '--tgt-image', 'foo-tgt-image',
         '--src-image', 'foo-src-image',
         '--output', 'foo-output',
-        '--check',
+        '--check', '--minios',
         '--private-key', 'foo-private-key',
         '--work-dir', 'foo-work-dir',
     ])
@@ -36,4 +36,4 @@
         src_image=partial_mock.HasString('foo-src-image'),
         work_dir=partial_mock.HasString('foo-work-dir'),
         private_key=partial_mock.HasString('foo-private-key'),
-        check=True)
+        check=True, minios=True)