Updates to avoid common pitfalls when changing the RELEASE variable

Fixes #635, fixes #645, fixes #632

stage0/prerun.sh

@@ -1,5 +1,10 @@
 #!/bin/bash -e
 
+if [ "$RELEASE" != "bullseye" ]; then
+	echo "WARNING: RELEASE does not match the intended option for this branch."
+	echo "         Please check the relevant README.md section."
+fi
+
 if [ ! -d "${ROOTFS_DIR}" ] || [ "${USE_QCOW2}" = "1" ]; then
 	bootstrap ${RELEASE} "${ROOTFS_DIR}" http://raspbian.raspberrypi.org/raspbian/
 fi