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.

export-noobs/00-release/00-run.sh

@@ -36,6 +36,7 @@ sed "${NOOBS_DIR}/partitions.json" -i -e "s|ROOT_NOM|${ROOT_NOM}|"
 sed "${NOOBS_DIR}/os.json" -i -e "s|UNRELEASED|${IMG_DATE}|"
 sed "${NOOBS_DIR}/os.json" -i -e "s|NOOBS_NAME|${NOOBS_NAME}|"
 sed "${NOOBS_DIR}/os.json" -i -e "s|NOOBS_DESCRIPTION|${NOOBS_DESCRIPTION}|"
+sed "${NOOBS_DIR}/os.json" -i -e "s|RELEASE|${RELEASE}|"
 
 sed "${NOOBS_DIR}/release_notes.txt" -i -e "s|UNRELEASED|${IMG_DATE}|"
 

export-noobs/00-release/files/os.json

@@ -16,5 +16,5 @@
     ],
     "url": "http://www.raspbian.org/",
     "username": "pi",
-    "version": "buster"
+    "version": "RELEASE"
 }