unmount cache before paving it over with the radio/hboot image

Try to unmount the cache.  If that fails, try to remount it read-only
and sync (which isn't as safe but is better than leaving it r/w).  If
we can't even do that, abort the firmware installation.

Change-Id: I2ebb1ebf3d1f58270133c58b0e9337fb33d78dfa