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.

stage0/00-configure-apt/00-run.sh

@@ -2,6 +2,8 @@
 
 install -m 644 files/sources.list "${ROOTFS_DIR}/etc/apt/"
 install -m 644 files/raspi.list "${ROOTFS_DIR}/etc/apt/sources.list.d/"
+sed -i "s/RELEASE/${RELEASE}/g" "${ROOTFS_DIR}/etc/apt/sources.list"
+sed -i "s/RELEASE/${RELEASE}/g" "${ROOTFS_DIR}/etc/apt/sources.list.d/raspi.list"
 
 if [ -n "$APT_PROXY" ]; then
 	install -m 644 files/51cache "${ROOTFS_DIR}/etc/apt/apt.conf.d/51cache"

stage0/00-configure-apt/files/raspi.list

@@ -1,3 +1,3 @@
-deb http://archive.raspberrypi.org/debian/ buster main
+deb http://archive.raspberrypi.org/debian/ RELEASE main
 # Uncomment line below then 'apt-get update' to enable 'apt-get source'
-#deb-src http://archive.raspberrypi.org/debian/ buster main
+#deb-src http://archive.raspberrypi.org/debian/ RELEASE main

stage0/00-configure-apt/files/sources.list

@@ -1,3 +1,3 @@
-deb http://raspbian.raspberrypi.org/raspbian/ buster main contrib non-free rpi
+deb http://raspbian.raspberrypi.org/raspbian/ RELEASE main contrib non-free rpi
 # Uncomment line below then 'apt-get update' to enable 'apt-get source'
-#deb-src http://raspbian.raspberrypi.org/raspbian/ buster main contrib non-free rpi
+#deb-src http://raspbian.raspberrypi.org/raspbian/ RELEASE main contrib non-free rpi

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