u-boot: Avoid breaking build for U-Boot-only binman images

Commit 46e01c0e1d ("u-boot: Avoid building U-Boot-only binman images")
added a patch that prevents an error while building U-Boot, due to some
U-Boot images needing a copy of BL31 that we are not passing in.

Removing build instructions for these images isn't really necessary,
when we can instead tell the build tool that it shouldn't exit with an
error. It checks a BINMAN_ALLOW_MISSING environment variable for this,
but just unconditionally replace the check with the argument.

Signed-off-by: Alper Nebi Yasak <alpernebiyasak@gmail.com>

config/u-boot/default/patches/0004-HACK-Makefile-Ignore-missing-input-files-for-binman.patch

@@ -0,0 +1,34 @@
+From ba34d29274c23c52be957ea040539dccbab09765 Mon Sep 17 00:00:00 2001
+From: Alper Nebi Yasak <alpernebiyasak@gmail.com>
+Date: Wed, 10 Jul 2024 17:37:56 +0300
+Subject: [PATCH] HACK: Makefile: Ignore missing input files for binman images
+
+For Rockchip boards binman tries to build SPI and MMC images that
+require an externally built BL31 file to be provided, and the build
+fails otherwise.
+
+Some downstreams only care about build outputs for U-Boot proper. As a
+hack to make sure they can do so without passing in a BL31 file, tell
+binman to ignore missing input files.
+
+Signed-off-by: Alper Nebi Yasak <alpernebiyasak@gmail.com>
+---
+ Makefile | 2 +-
+ 1 file changed, 1 insertion(+), 1 deletion(-)
+
+diff --git a/Makefile b/Makefile
+index 1c754ceb5953..fec8d9b2d2b1 100644
+--- a/Makefile
++++ b/Makefile
+@@ -1375,7 +1375,7 @@ cmd_binman = $(srctree)/tools/binman/binman $(if $(BINMAN_DEBUG),-D) \
+                 --toolpath $(objtree)/tools \
+ 		$(if $(BINMAN_VERBOSE),-v$(BINMAN_VERBOSE)) \
+ 		build -u -d u-boot.dtb -O . -m \
+-		--allow-missing $(if $(BINMAN_ALLOW_MISSING),--ignore-missing) \
++		--allow-missing --ignore-missing \
+ 		-I . -I $(srctree) -I $(srctree)/board/$(BOARDDIR) \
+ 		-I $(dt_dir) -a of-list=$(CONFIG_OF_LIST) \
+ 		$(foreach f,$(BINMAN_INDIRS),-I $(f)) \
+-- 
+2.45.2
+