Review request for the fix to generate jmod files for jdk modules

[Mandy Chung]

Hi Dalibor,

Can you review the change to generate jmod files for
jdk modules?

Webrev:
  http://cr.openjdk.java.net/~mchung/jigsaw/gen-jmod-files/

The modules build will generate jmod files under
$outputdir/jigsaw-pkgs/jmod directory.  The debian
packages are in $outputdir/jigsaw-pkgs/deb.

It calls jpkg to create the jmod files for jdk modules.
I temporarily disable ensureShortNativePath check in
ModuleFileFormat.java until we make the jdk changes
to conform the module file format spec (i.e. the path
names of native-code files must not include more than
one element and no execute permission set).

include header files are in the jdk.boot module and
only copied to jdk-base-image and jdk-module-image.
In addition, I clean up Modules.gmk as there is no reason
to have the jre subdirectory in the jdk-module-image
and jdk-base-image.  They should be the same as 
jre-base-image with a set of jdk modules installed.

Thanks
Mandy