[openwrt/openwrt] ramips: fix wrg-header image recipe

[LEDE Commits]

robimarko pushed a commit to openwrt/openwrt.git, branch main:
https://git.openwrt.org/e247b7ab66c3a754c652e715c117c150d6598814

commit e247b7ab66c3a754c652e715c117c150d6598814
Author: Shiji Yang <yangshiji66 at outlook.com>
AuthorDate: Sun May 18 20:42:19 2025 +0800

    ramips: fix wrg-header image recipe
    
    Before generating the factory image, check if the input file
    exists. Fix the build error when sysupgrade image is too big:
    
    [mkwrgimg] *** error: stat failed on /builder/shared-workdir/build/build_dir/target-mipsel_24kc_musl/linux-ramips_rt288x/tmp/openwrt-ramips-rt288x-airlink101_ar670w-squashfs-factory.bin, No such file or directory
    
    Signed-off-by: Shiji Yang <yangshiji66 at outlook.com>
    Link: https://github.com/openwrt/openwrt/pull/18836
    Signed-off-by: Robert Marko <robimarko at gmail.com>
---
 target/linux/ramips/image/Makefile | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/target/linux/ramips/image/Makefile b/target/linux/ramips/image/Makefile
index 53b9c314ef..4d7f65320c 100644
--- a/target/linux/ramips/image/Makefile
+++ b/target/linux/ramips/image/Makefile
@@ -163,7 +163,8 @@ define Build/umedia-header
 endef
 
 define Build/wrg-header
-	mkwrgimg -i $@ -d "/dev/mtdblock/2" -s $(1) -o $@.new
+	-[ -f "$@" ] && \
+	mkwrgimg -i $@ -d "/dev/mtdblock/2" -s $(1) -o $@.new && \
 	mv $@.new $@
 endef