6674232: OPENJDK=false is same as OPENJDK=true

OPENJDK should be empty (undefined) or "true".

Reviewed-by: tbell

jdk/make/common/Defs.gmk

@@ -32,6 +32,13 @@
 #          So when it includes other files, it must use JDK_TOPDIR.
 #
 
+# Check for strange explicit settings (change to empty or true)
+ifdef OPENJDK
+  ifneq ($(OPENJDK),true)
+    x:=$(error "OPENJDK (if defined) can only be set to true")
+  endif
+endif
+
 #
 # On Solaris, the 'make' utility from Sun will not work with these makefiles.
 #    This little rule is only understood by Sun's make, and is harmless
@@ -87,19 +94,6 @@ ifeq ($(CLOSED_SRC_DIR_EXISTS), false)
   OPENJDK = true
 endif
 
-# Check for strange explicit settings (change to empty or true)
-ifdef OPENJDK
-  ifeq ($(OPENJDK),false)
-    # Silently treat as not defined
-    OPENJDK =
-  else
-    ifneq ($(OPENJDK),true)
-      dummy := $(warning "WARNING: OPENKJDK=$(OPENJDK) being treated as true")
-      OPENJDK = true
-    endif
-  endif
-endif
-
 # Define where closed directories are
 ifdef OPENJDK
   CLOSED_SRC =