Fix release version fallbacks and enable attribute substitution

Author: shai-almog

docs/developer-guide/Maven-Getting-Started.adoc

@@ -210,7 +210,7 @@ Let's consider a concrete example, now.  Download the KitchenSink Ant project fr
 
 The following is a bash script that uses curl to download this project as a zip file, and then converts it to a fully-functional Maven project.
 
-[source,bash]
+[source,bash,subs="+attributes"]
 ----
 CN1_VERSION={cn1-plugin-release-version}
 curl -L https://github.com/codenameone/KitchenSink/archive/v1.0-cn7.0.11.zip > master.zip

docs/developer-guide/developer-guide.asciidoc

@@ -28,15 +28,11 @@
 :copyright: Codename One, all rights reserved
 :publication-type: book
 :producer: Codename One Ltd.
-:cn1-release-version: {env:CN1_RELEASE_VERSION}
-ifeval::["{cn1-release-version}" == ""]
-:cn1-release-version: 7.0.21
-endif::[]
+:cn1-release-version: 7.0.210
+ifdef::env-CN1_RELEASE_VERSION[:cn1-release-version: {env:CN1_RELEASE_VERSION}]
 
-:cn1-plugin-release-version: {env:CN1_PLUGIN_RELEASE_VERSION}
-ifeval::["{cn1-plugin-release-version}" == ""]
 :cn1-plugin-release-version: {cn1-release-version}
-endif::[]
+ifdef::env-CN1_PLUGIN_RELEASE_VERSION[:cn1-plugin-release-version: {env:CN1_PLUGIN_RELEASE_VERSION}]
 
 :cn1-snapshot-version: {cn1-release-version}-SNAPSHOT