RFR: JDK-8033119 Improve and document boot-jdk.m4

Wed Jan 29 12:06:06 UTC 2014

Bug: https://bugs.openjdk.java.net/browse/JDK-8033119

The file boot-jdk.m4 is in charge of locating and verifying the Boot JDK.

This bug covers several independent fixes of that file:
* Document the logic of how we discover a suitable Boot JDK.
* Update to allow building with JDK 9.
* Improve detection on macosx by preferring the official method.
* Allow overriding of individual tools normally found in the Boot JDK 
(e.g. JAVA, JAVAC)
* Update list of tools: RMIC is not used anymore, JARSIGNER was not 
tested for properly.

WebRev: 
http://cr.openjdk.java.net/~ihse/JDK-8033119-improve-and-document-boot-jdk/webrev.01

/Magnus