image_types.bbclass: Use --force also with lz4,lzop

image_types.bbclass: Use --force also with lz4,lzop

Several conversion commands already make use of 'force' option in the
compression, which enables overwriting existing files without
prompting.

Since occasionally an existing residual destination file from a
previously aborted or failed task can prevent the re-execution of the
conversion command task, by enabling the 'force' option also for lz4
and lzop compression commands we can avoid following kind of BitBake
failures with these compressors:

  | DEBUG: Executing shell function do_image_cpio
  | 117685 blocks
  | 2 blocks
  | example-image.cpio.lz4 already exists; do you want to overwrite (y/N) ?     not overwritten
  | Error 20 : example-image.cpio : open file error
  | WARNING: exit code 20 from a shell command.
  ERROR: Task (.../recipes-core/images/example-image.bb:do_image_cpio) failed with exit code '1'

Signed-off-by: Niko Mauno <niko.mauno@vaisala.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>