bootm: make use of legacy image format configurable

bootm: make use of legacy image format configurable

make the use of legacy image format configurable through
the config define CONFIG_IMAGE_FORMAT_LEGACY.

When relying on signed FIT images with required signature check
the legacy image format should be disabled. Therefore introduce
this new define and enable legacy image format if CONFIG_FIT_SIGNATURE
is not set. If CONFIG_FIT_SIGNATURE is set disable per default
the legacy image format.

Signed-off-by: Heiko Schocher <[email protected]>
Cc: Simon Glass <[email protected]>
Cc: Lars Steubesand <[email protected]>
Cc: Mike Pearce <[email protected]>
Cc: Wolfgang Denk <[email protected]>
Cc: Tom Rini <[email protected]>
Cc: Michal Simek <[email protected]>
Acked-by: Simon Glass <[email protected]>