avbtool: Use root=/dev/dm-0 instead of root=0xfd00.

It was pointed out that major/minor numbers could be dynamically
allocated. While this is unlikely, just use /dev/dm-0 which is
guaranteed to always refer to the first mapped device.

Bug: 36444660
Test: All unit tests pass.
Test: Manually tested on UEFI based bootloader.
Change-Id: If591fae1ee657ab6da0ab9b140e45965104da3fd
3 files changed