If building with SPL_LOAD_FIT_FULL and FIT_SIGNATURE, but without
SPL_FIT_SIGNATURE then the build fails with:

  common/built-in.o: In function `fit_image_verify_with_data':
  common/image-fit.c:1220: undefined reference to `fit_image_verify_required_sigs'
  common/image-fit.c:1244: undefined reference to `fit_image_check_sig'
  common/built-in.o: In function `fit_image_load':
  common/image-fit.c:1857: undefined reference to `fit_config_verify'

Refactor so that host builds still depend on FIT_SIGNATURE, but target
builds check FIT_SIGNATURE/SPL_FIT_SIGNATURE dependent on the build.
Signed-off-by: NAlex Kiernan <alex.kiernan@gmail.com>
Reviewed-by: NSimon Glass <sjg@chromium.org>