Improve and document boot-jdk.m4

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.