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

[Erik Joelsson]

Hello,

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/

/Erik