dual-image: Mount alt dirs

To support dual image in static layout, it needs to mount secondary's
rofs/rwfs.

Update obmc-flash-bmc tool to support mounting 'alt-xxx' directories in
/run/media/alt-xxx, and try to mount the alt-rofs, alt-rwfs before
xyz.openbmc_project.Software.BMC.Updater.service starts.

The ExecStart command is prepended with `-` so that it does not matter
if the mount fails, because the alt chip may be empty or contain invalid
data.

Tested: Verify the obmc-flash-bmc-static-mount-alt.service starts before
        xyz.openbmc_project.Software.BMC.Updater.service, and verify it
        mounts alt-rofs for dual image systems.
        The alt-rwfs will be mounted by the needed service in future
        commits.

Signed-off-by: Lei YU <yulei.sh@bytedance.com>
Change-Id: I2133a99d5324a1008c3508154f865ae54a716219
4 files changed