RFR: JDK-8056062: Additional minor cleanups from source restructure build changes

[Tim Bell]

Erik:

> Please review this small patch fixing a couple of issues that have 
> been raised on this list following the source code restructure.
>
> * Fix spelling error in make help (build->built)
> * Readd the jdk target as an alias for exploded-image as people are 
> used to the target and the error message when it's missing is confusing.
> * Print a warning when clean targets and other targets are mixed on 
> the command line as this won't work well. Also add a note about this 
> in the help target.
>
> Bug: https://bugs.openjdk.java.net/browse/JDK-8056062
> Webrev: http://cr.openjdk.java.net/~erikj/8056062/webrev.root.01

Looks good to me.  Approved.

Tim