f2fs: Update make rules for CM compatibility

* Make sure fsck.f2fs and mkfs.f2fs are built for CM12.1/13.0
* Add CM13.0 rule for relinking mkfs.f2fs
* Minor cleanup of nested if/else in prebuilts

Note on CM SDKs: 3=cm-12.1, 4=cm-13.0
I'm not yet sure whether it is possible that the CMSDK increments
within the same branch of CM.

Change-Id: I7b521159a8b5d0e0ea639f5e0fdd885100612df0
diff --git a/Android.mk b/Android.mk
index 61435c3..6d9959c 100644
--- a/Android.mk
+++ b/Android.mk
@@ -415,6 +415,13 @@
         ntfsfix \
         mkntfs
 endif
+ifeq ($(TARGET_USERIMAGES_USE_F2FS), true)
+ifeq ($(shell test $(CM_PLATFORM_SDK_VERSION) -ge 3; echo $$?),0)
+    LOCAL_ADDITIONAL_DEPENDENCIES += \
+        fsck.f2fs \
+        mkfs.f2fs
+endif
+endif
 
 include $(BUILD_EXECUTABLE)