Improve update_issue() function in common script

* Allow to customise pi-gen script name and repository in the generated issue file
* Preserve compatibility with existing function calls

build.sh

@@ -137,6 +137,9 @@ do
 	esac
 done
 
+export PI_GEN=${PI_GEN:-pi-gen}
+export PI_GEN_REPO=${PI_GEN_REPO:-https://github.com/RPi-Distro/pi-gen}
+
 if [ -z "${IMG_NAME}" ]; then
 	echo "IMG_NAME not set" 1>&2
 	exit 1

scripts/common

@@ -96,7 +96,7 @@ export -f on_chroot
 update_issue() {
 	local GIT_HASH
 	GIT_HASH=$(git rev-parse HEAD)
-	echo -e "Raspberry Pi reference ${IMG_DATE}\nGenerated using pi-gen, https://github.com/RPi-Distro/pi-gen, ${GIT_HASH}, ${1}" > "${ROOTFS_DIR}/etc/rpi-issue"
+	echo -e "Raspberry Pi reference ${IMG_DATE}\nGenerated using ${PI_GEN}, ${PI_GEN_REPO}, ${GIT_HASH}, ${1}" > "${ROOTFS_DIR}/etc/rpi-issue"
 }
 export -f update_issue