Fix build bash.

builder/build.sh

@@ -178,6 +178,7 @@ export TARGET_HOSTNAME=${TARGET_HOSTNAME:-raspberrypi}
 
 export FIRST_USER_NAME=${FIRST_USER_NAME:-pi}
 export FIRST_USER_PASS=${FIRST_USER_PASS:-raspberry}
+export RELEASE=${RELEASE:-buster}
 export WPA_ESSID
 export WPA_PASSWORD
 export WPA_COUNTRY

builder/stage0/prerun.sh

@@ -2,4 +2,4 @@
 
 if [ ! -d "${ROOTFS_DIR}" ]; then
 	bootstrap ${RELEASE} "${ROOTFS_DIR}" http://raspbian.raspberrypi.org/raspbian/
-fi
+fi