u-boot: Relax KCFLAGS warnings for GCC 15

Append -Wno-error flags for int-conversion, implicit function
declarations, and format strings to KCFLAGS.

Tested:
Verified build with GCC 15 and C++23 across gBMC platforms.
- platform11: http://fusion2/3ff2006d-9c03-3f0c-99fd-bcefc6aaca1f
- platform11: http://fusion2/982d88b5-28f9-38ef-a3a2-d824b919c13c
- platform5: http://fusion2/a7e89bcd-86a0-33b2-8f33-2bb24ef3afb5
- platform15: http://fusion2/e56dcb19-6dcd-35a0-b5ee-30740fbf27df
- platform17: http://fusion2/d0e6af61-eb95-3a70-a6a3-d1bdab987ea2
- platform18: http://fusion2/cda6ba3a-58f5-3907-b584-04786b9fe634
- platform25: http://fusion2/714cc620-6f0a-3bfd-90dd-95356b62b54b
Google-Bug-Id: 482144905
Change-Id: I7c0884ce11457bc6f6931486a4bf562f9b117234
Signed-off-by: Willy Tu <wltu@google.com>
diff --git a/dynamic-layers/aspeed-layer/recipes-bsp/u-boot/u-boot-aspeed-sdk_%.bbappend b/dynamic-layers/aspeed-layer/recipes-bsp/u-boot/u-boot-aspeed-sdk_%.bbappend
index fae379b..daa5d32 100644
--- a/dynamic-layers/aspeed-layer/recipes-bsp/u-boot/u-boot-aspeed-sdk_%.bbappend
+++ b/dynamic-layers/aspeed-layer/recipes-bsp/u-boot/u-boot-aspeed-sdk_%.bbappend
@@ -16,3 +16,10 @@
 SRC_URI:append:gbmc:aspeed-g7 = " \
     file://0001-u-boot-adjust-memory-layout-for-ast2700.patch \
 "
+
+EXTRA_OEMAKE:append:gbmc = " KCFLAGS='-Wno-error=int-conversion \
+  -Wno-error=implicit-function-declaration \
+  -Wno-error=format \
+  -Wno-error=return-type' \
+"
+
diff --git a/dynamic-layers/nuvoton-layer/recipes-bsp/u-boot/u-boot-nuvoton_%.bbappend b/dynamic-layers/nuvoton-layer/recipes-bsp/u-boot/u-boot-nuvoton_%.bbappend
index 0baa0bd..344ea49 100644
--- a/dynamic-layers/nuvoton-layer/recipes-bsp/u-boot/u-boot-nuvoton_%.bbappend
+++ b/dynamic-layers/nuvoton-layer/recipes-bsp/u-boot/u-boot-nuvoton_%.bbappend
@@ -7,3 +7,7 @@
     file://0004-recipe-uboot-WA-to-disable-GFX-module.patch \
     file://0005-misc-npcm_host_intf-Disable-pending-KCS-BPC-interrup.patch \
   "
+EXTRA_OEMAKE:append:gbmc = " KCFLAGS='-Wno-error=int-conversion \
+  -Wno-error=implicit-function-declaration \
+  -Wno-error=format' \
+"