Support /dev/mapper/rootfs root device

Support /dev/mapper/rootfs root device

Root device is /dev/mapper/rootfs on RPI4 with dm-verity.

So, the actual root device is not correctly found,
and /dev/mmcblk0 is not recognized as an internal device.

Storaged recognizes the partitions of /dev/mmcblk0p* as external devices,
and storaged mounts them to /opt/media/SDCard@*.

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/dev/mmcblk0p1     65390  20830   44560  32% /opt/media/SDCard@1
/dev/mmcblk0p10    59365   1299   53480   3% /opt/media/SDCard@10
/dev/mmcblk0p11   120355   1555  109805   2% /opt/media/SDCard@11
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Change-Id: I42f211684e519371a23ee0b403bd3d941d243bb0