global: Do not default to faking missing binaries for buildman

While it is possible and documented on how to re-run buildman to replace
faked required binary files after the fact, this behavior ends up being
more confusing than helpful in practice. Switch to requiring
BINMAN_ALLOW_MISSING=1 to be passed on the 'make' line to enable this
behavior.

Cc: Rasmus Villemoes <rasmus.villemoes@prevas.dk>
Cc: Simon Glass <sjg@chromium.org>
Signed-off-by: Tom Rini <trini@konsulko.com>
Signed-off-by: Simon Glass <sjg@chromium.org>

Makefile

@@ -1333,8 +1333,8 @@ cmd_binman = $(srctree)/tools/binman/binman $(if $(BINMAN_DEBUG),-D) \
 		$(foreach f,$(BINMAN_TOOLPATHS),--toolpath $(f)) \
                 --toolpath $(objtree)/tools \
 		$(if $(BINMAN_VERBOSE),-v$(BINMAN_VERBOSE)) \
-		build -u -d u-boot.dtb -O . -m --allow-missing \
-		--fake-ext-blobs \
+		build -u -d u-boot.dtb -O . -m \
+		$(if $(BINMAN_ALLOW_MISSING),--allow-missing --fake-ext-blobs) \
 		-I . -I $(srctree) -I $(srctree)/board/$(BOARDDIR) \
 		-I arch/$(ARCH)/dts -a of-list=$(CONFIG_OF_LIST) \
 		$(foreach f,$(BINMAN_INDIRS),-I $(f)) \