RFR: JDK-8244021 Hide warning from jlink about incubating modules

[Magnus Ihse Bursie]

The last remaining "WARNING" that is printed while building the JDK 
stems from jlink. This is due to the fact that jmod is run with the flag 
--warn-if-resolved=incubating, which will cause future processing of the 
jmod to print a warning.

This is by design to get the warning to display for the end users.

However, this also has the side-effect that our call to jlink will 
display this warning, unconditionally. This warning from jlink cannot be 
disabled, per design.

Since we know that we have incubating modules from time to time, and it 
is not a warning for us, and the message cannot be removed using flags 
to jlink, I propose to just filter it out, so we can achieve a fully 
warning-free build.

The filtering can always be disabled by running like this "make images 
JLINK_DISABLE_WARNINGS="

Bug: https://bugs.openjdk.java.net/browse/JDK-8244021
Patch inline:
diff --git a/make/Images.gmk b/make/Images.gmk
--- a/make/Images.gmk
+++ b/make/Images.gmk
@@ -85,6 +85,8 @@
    JLINK_JDK_EXTRA_OPTS := --keep-packaged-modules $(JDK_IMAGE_DIR)/jmods
  endif

+JLINK_DISABLE_WARNINGS := | ( $(GREP) -v -e "WARNING: Using incubator 
module" || test "$$?" = "1" )
+
  $(eval $(call SetupExecute, jlink_jdk, \
      WARN := Creating jdk image, \
      DEPS := $(JMODS) $(BASE_RELEASE_FILE) \
@@ -93,7 +95,8 @@
      SUPPORT_DIR := $(SUPPORT_OUTPUTDIR)/images/jdk, \
      PRE_COMMAND := $(RM) -r $(JDK_IMAGE_DIR), \
      COMMAND := $(JLINK_TOOL) --add-modules $(JDK_MODULES_LIST) \
-        $(JLINK_JDK_EXTRA_OPTS) --output $(JDK_IMAGE_DIR), \
+        $(JLINK_JDK_EXTRA_OPTS) --output $(JDK_IMAGE_DIR) \
+        $(JLINK_DISABLE_WARNINGS), \
  ))