Urgent (S) RFR: 8155689: Build failing for install on jdk9/dev when using -testset hotspot

[David Holmes]

Bug report is not public as it relates to issues with our internal jib 
builds, but the fix for this needs a tweak to the open Main.gmk file.

This is a follow up to the fix a couple of days ago for

JDK-8154841: Let different Jib profiles have different default make targets

A side-effect of that change was that when a jib build is done with our 
closed sources present the default make target is set to "installer". 
But there is no such target in the open Main.gmk file so we get a make 
error.

Simple solution is to add one that is equivalent to the default make 
target set by the open jib code:

http://cr.openjdk.java.net/~dholmes/8155689/webrev/make/Main.gmk.cdiff.html

+
+ # workaround issue when building open targets when closed 
jib-profiles.js is used
+ installer: product-images test-image
+

Tested in JPRT using non -testset hotspot, and -testset hotspot (the 
latter is what exposed the problem).

Thanks,
David