RFR: JDK-8188814 Simplify IncludeCustomExtension

[Magnus Ihse Bursie]

Now that we only have a single repo, the first argument to 
IncludeCustomExtension is no longer needed.

I looked at some more ambitious ways to simplify IncludeCustomExtension. 
Getting make to automatically retrieve the name of the file is just 
sooooo tantalizingly close to possible, but ultimately not doable. :-(

One way to achieve this (and also get automatic hooks for custom 
extensions prior and after including a file) would be to replace all 
places now sporting "include $FILE" with a "$(eval $(call include, 
$FILE))". This would also help somewhat with debuggability, by providing 
means from the code, e.g. in error messages, to know the name of the 
current file, and showing the include tree in e.g. LOG=debug.

However, in the end, I decided it was not really worth it. We have 200+ 
includes that needed changing, and then we would need to remember to use 
this pattern for all new future changes.

Bug: https://bugs.openjdk.java.net/browse/JDK-8188814
WebRev: 
http://cr.openjdk.java.net/~ihse/JDK-8188814-simplify-IncludeCustomExtension/webrev.01

/Magnus