RFR: JDK-8073498: Enhance GensrcProperties.gmk to allow an alternative source root

[Erik Joelsson]

Hello,

Please review these two small fixes to the build system. When we tried 
to add a custom Gensrc-<module>.gmk file, it didn't work as expected. 
The -I flags on the make command line were wrong, so it couldn't import 
the GensrcCommon.gmk or GensrcProperties.gmk from jdk/make/gensrc. Also, 
GensrcProperties.gmk did not support generating java files from 
properties in a different source root than jdk/src/<module>.

Bug: https://bugs.openjdk.java.net/browse/JDK-8073498
Webrev: http://cr.openjdk.java.net/~erikj/8073498/webrev.01/

These changes have been tested with the patch for JDK-8062750, through 
which we discovered these shortcomings.

/Erik