ci: build: Patch Raspberry Pi kernel sources

The location of the Raspberry Pi kernel source specified in the
project's configuration must match exactly the one provided by
buildroot, in order to avoid a `No hash found for linux-....tar.gz`
error at build time.  To ensure this is always the case, without having
to update the project configuration for every Buildroot update, we patch
the configuration at build time with the current value from the
Buildroot default configuration.

ci/build.sh

@@ -13,6 +13,13 @@ else
     git -C buildroot merge FETCH_HEAD --ff-only
 fi
 
+sed -i \
+    -e /BR2_LINUX_KERNEL_CUSTOM_TARBALL/a"$(
+        grep BR2_LINUX_KERNEL_CUSTOM_TARBALL_LOCATION= buildroot/configs/raspberrypi3_defconfig
+    )" \
+    -e /BR2_LINUX_KERNEL_CUSTOM_TARBALL_LOCATION/d \
+    configs/"${DEFCONFIG}"
+
 if [ ! -f _build/.config ]; then
     make -C buildroot O="${PWD}"/_build BR2_EXTERNAL="${PWD}" "${DEFCONFIG}"
 fi