fastboot: Prefer ro.boot.hardware.revision for hw-revision

ro.revision defaults to 0 if it's not set in cmdline.
Some devices might want to set it in their device specific init files,
however it's not possible to override ro properties.

Test: Set ro.boot.hardware.revision in device specific init.rc and
      observe fastboot getvar hw-revision in fastbootd

Change-Id: I6e785c3fe3a49409e815af269a9a8db732b80ada