mkf2fsuserimg.sh: Make sparse mode optional

Bug: 142424832
Bug: 142803849
Test: Run mkf2fsuserimg.sh without the sparse option and check that the
      generated image is not sparse.
Co-authored-by: Satya Tangirala <satyat@google.com>
Change-Id: I8a079b30f63ddd13a94d73339501ed8ecb2c5049
1 file changed