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The implementation of make_ext4fs() calls make_ext4fs_internal() by
forcing the |wipe| parameter to true, which is problematic when the
library is used within the emulator (the wipe operation will always
fail on non-Linux platforms).

This patch does the following:

- Add a 'is_block_device_fd()' function to check that a file descriptor
  points to a real block device.

- Modify the implementation of wipe_block_device() uses it to return
  silently when trying to wipe a non-block-device file.

- Add a WIPE_IS_SUPPORTED flag in wipe.h that indicates whether
  block device wiping is supported on the current platform
  (for now, this is only the case on Linux).

BUG=NONE

Change-Id: I62b62b7c3e99b465c3b876154231e7c2fe541b23