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The param is just ignored when the image is squashfs, but setting
"rw" for the read-only fs is still confusing.

This also adds a test that covers the case where no serial number
is given.

BUG=None
TEST=try

Change-Id: Id153e51f4d44432728e7735620853833f784a82a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1931919
Commit-Queue: Yusuke Sato <yusukes@chromium.org>
Reviewed-by: Hidehiko Abe <hidehiko@chromium.org>
Cr-Commit-Position: refs/heads/master@{#718808}