Only allow disabling SELinux on userdebug / eng builds

Only parse and honor the kernel command line on userdebug
or eng builds. On user builds, assume that selinux is always enabled
and enforcing.

Change-Id: I71c66e4365bdf2f226800634126a38b716d96599
diff --git a/init/Android.mk b/init/Android.mk
index 15a23be..489dc93 100644
--- a/init/Android.mk
+++ b/init/Android.mk
@@ -25,7 +25,7 @@
 endif
 
 ifneq (,$(filter userdebug eng,$(TARGET_BUILD_VARIANT)))
-LOCAL_CFLAGS += -DALLOW_LOCAL_PROP_OVERRIDE=1
+LOCAL_CFLAGS += -DALLOW_LOCAL_PROP_OVERRIDE=1 -DALLOW_DISABLE_SELINUX=1
 endif
 
 # Enable ueventd logging