RFR: JDK-8206311: Add docs-javase, docs-reference to CI build

[Erik Joelsson]

In order to regularly build the javase and reference docs images, I have 
made the following changes:

1. Introduced bundle targets for the javase and reference docs images.
2. To run this in CI without adding more unnecessary build time to the 
linux-x64 target (where the current docs are built), I created a new jib 
profile "docs". This will run in a separate task and will build all 3 
docs targets.
3. To enable the docs targets to run standalone without having to build 
a full JDK, I had to tweak the "build-jdk" concept a bit, to allow it to 
also function when not cross compiling for certain parts of the build. 
Specifically the buildtools-modules can now be built by a "build-jdk" 
instead of the exploded image.
4. For symmetry, I renamed variables and targets involving docs images 
to always include one of "jdk", "javase" or "reference" in the name. See 
below for targets.

The new top level targets are:

docs-{jdk.javase,reference}-image:
Aliases for the existing docs-{jdk,javase,reference} added for 
consistency. The old docs-image is now an alias for docs-jdk-image.

all-docs-images:
Builds all 3 of the above docs images.

docs-{jdk,javase,reference}-bundles:
Builds the bundles for each kind of docs image. The existing 
docs-bundles is now an alias for docs-jdk-bundles.

all-docs-bundles:
Builds all the docs bundles.

Bug: https://bugs.openjdk.java.net/browse/JDK-8206311

Webrev: http://cr.openjdk.java.net/~erikj/8206311/webrev.01/index.html

/Erik