RFR: JDK-8179105 A new property to specify import module to be included in unified docs bundle

[Magnus Ihse Bursie]

javafx modules include a `build.properties` file to specify the properties
when being imported in JDK image build. `make docs` currently includes 
all imported modules.

  A new property e.g. `include_in_docs` would allow FX to select which
FX modules to be included in the docs bundle.

Bug: https://bugs.openjdk.java.net/browse/JDK-8179105
Patch inline:
diff --git a/make/common/Modules.gmk b/make/common/Modules.gmk
--- a/make/common/Modules.gmk
+++ b/make/common/Modules.gmk
@@ -391,7 +391,10 @@
        else ifeq ($$(classloader), ext)
          PLATFORM_MODULES += $1
        endif
-      DOCS_MODULES += $1
+      ifneq ($$(include_in_docs), false)
+        # defaults to true if unspecified
+        DOCS_MODULES += $1
+      endif
      else
        # Default to include in all
        JRE_MODULES += $1

/Magnus