image_type_phosphor: fix signatures generation

- We will hit a problem intermittently when we build image-bmc:
  We have 2 tasks running in parallel, the `do_generate_static` and
  `do_generate_static_alltar`. Both specify to use the same work
  directory, and both call to generate the signature files:

  do_generate_static_tar[dirs] = " ${S}/static"
  do_generate_static_alltar[dirs] = "${S}/static"

  Depending on timing, one task can remove the `image-full` file from
  the directory and the other task won't find it, and cause built
  failed.

  Error message: image-full: No such file or directory.

- Resolves openbmc/openbmc#3797

Tested: built obmc-phopshor-image successfully.

Temporarily solve openbmc/issues/3797

Signed-off-by: George Liu <liuxiwei@inspur.com>
Change-Id: I7147f6c8db4f3a2a0c0c9bd54ac384ee8882b233

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