Makefile: Warn against using CONFIG_SPL_FIT_GENERATOR

This option is used to run arch-specific shell scripts which produce .its
files which are used to produce FIT images. We already have binman which
is designed to produce firmware images. It is more powerful and has tests.

So this option should be deprecated and not used. Existing uses should be
migrated.

Mentions of this in code reviews over the last year or so do not seem to
have resulted in action, and things are getting worse.

So let's add a warning.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>

Makefile

@@ -1148,6 +1148,13 @@ ifneq ($(CONFIG_DM_ETH),y)
 	@echo >&2 "See doc/driver-model/migration.rst for more info."
 	@echo >&2 "===================================================="
 endif
+endif
+ifneq ($(CONFIG_SPL_FIT_GENERATOR),)
+	@echo >&2 "===================== WARNING ======================"
+	@echo >&2 "This board uses CONFIG_SPL_FIT_GENERATOR. Please migrate"
+	@echo >&2 "to binman instead, to avoid the proliferation of"
+	@echo >&2 "arch-specific scripts with no tests."
+	@echo >&2 "===================================================="
 endif
 	@# Check that this build does not use CONFIG options that we do not
 	@# know about unless they are in Kconfig. All the existing CONFIG
@@ -1345,6 +1352,8 @@ endif
 
 # Boards with more complex image requirements can provide an .its source file
 # or a generator script
+# NOTE: Please do not use this. We are migrating away from Makefile rules to use
+# binman instead.
 ifneq ($(CONFIG_SPL_FIT_SOURCE),"")
 U_BOOT_ITS := u-boot.its
 $(U_BOOT_ITS): $(subst ",,$(CONFIG_SPL_FIT_SOURCE))