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[Marc Kleine-Budde]

Hello,

the first patch cleans up the error message output if struct
image_type_params::set_header is set, the other adds support for
creating legacy images with type flat_dt.

regards,
Marc

Changes since v2:
- renamed new image type to "fdt_legacy" (suggested by Sean Anderson)
Changes since v1:
- introduce new image type (suggested by Sean Anderson)
  instead of adding new cmd line parameter