item_updater: sort ubi specific code

Add item_updater_helper and implement it in ubi and static layouts.

Tested: Build this repo on both Romulus and Witherspoon OK;
        Tested on Romulus and Witherspoon that code update works fine.

Change-Id: I706cfe63900f89bb41672dcc745b3483e06838c7
Signed-off-by: Lei YU <mine260309@gmail.com>
diff --git a/ubi/Makefile.am.include b/ubi/Makefile.am.include
index 7bcfcb2..1a5128a 100644
--- a/ubi/Makefile.am.include
+++ b/ubi/Makefile.am.include
@@ -1,2 +1,3 @@
 phosphor_image_updater_SOURCES += \
-	%reldir%/flash.cpp
+	%reldir%/flash.cpp \
+	%reldir%/item_updater_helper.cpp