Refactor IMAGE_ENABLE_VERIFY to handle builds without SPL verification

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: Alex Kiernan <alex.kiernan@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/include/image.h b/include/image.h
index 889305c..54f2b58 100644
--- a/include/image.h
+++ b/include/image.h
@@ -1072,18 +1072,18 @@
  * At present we only support signing on the host, and verification on the
  * device
  */
-#if defined(CONFIG_FIT_SIGNATURE)
-# ifdef USE_HOSTCC
+#if defined(USE_HOSTCC)
+# if defined(CONFIG_FIT_SIGNATURE)
 #  define IMAGE_ENABLE_SIGN	1
 #  define IMAGE_ENABLE_VERIFY	1
-# include  <openssl/evp.h>
-#else
+#  include <openssl/evp.h>
+# else
 #  define IMAGE_ENABLE_SIGN	0
-#  define IMAGE_ENABLE_VERIFY	1
+#  define IMAGE_ENABLE_VERIFY	0
 # endif
 #else
 # define IMAGE_ENABLE_SIGN	0
-# define IMAGE_ENABLE_VERIFY	0
+# define IMAGE_ENABLE_VERIFY	CONFIG_IS_ENABLED(FIT_SIGNATURE)
 #endif
 
 #ifdef USE_HOSTCC