Exposed RELEASE as a configurable variable (#391)

Exposed RELEASE as a configurable variable and updated README.md.  As a
bonus, there is only on place to change the release name when the next
release is stable now.

stage0/prerun.sh

@@ -1,5 +1,5 @@
 #!/bin/bash -e
 
 if [ ! -d "${ROOTFS_DIR}" ]; then
-	bootstrap buster "${ROOTFS_DIR}" http://raspbian.raspberrypi.org/raspbian/
+	bootstrap ${RELEASE} "${ROOTFS_DIR}" http://raspbian.raspberrypi.org/raspbian/
 fi