RFR(S): 7901304: IDPREFIX should be configurable from the environment much like BUGURL already is

[Volker Simonis]

Hi,

could somebody please review and sponsor the following small change:

http://cr.openjdk.java.net/~simonis/webrevs/2015/7901304/
https://bugs.openjdk.java.net/browse/CODETOOLS-7901304

It is possible to configure the bug url for links to the corresponding
OpenJDK bugs by setting the environment variable WEBREV_BUGURL.

Unfortunately it is not possible to configure the IDPREFIX which is
hard-wired to "JDK-". This makes it impossible to create links to the
bug systems for projects like CODETOOLS.

The fix is easy - just read IDPREFIX from the environment if there
exists an environment variable called WEBREV_IDPREFIX (by the way, the
result can be seen in the webrev which correctly links to the
corresponding codetools bug :).

I've also slightly reordered the place where WEBREV_BUGURL and
WEBREV_IDPREFIX are initialized to make it possible to print their
default values in the webrev usage text.

Regards,
Volker