Refactor: Move serialize to ubi

Functions in serialize are specific to UBI, move it to ubi dir.

Tested: On the last commit of the patch series, run code update and
        factory reset on Witherspoon and all work fine.

Change-Id: I9176e638d9f3bfe0d424b57f8da4667a751cb5bd
Signed-off-by: Lei YU <mine260309@gmail.com>
diff --git a/ubi/Makefile.am.include b/ubi/Makefile.am.include
index c75293d..decf1e9 100644
--- a/ubi/Makefile.am.include
+++ b/ubi/Makefile.am.include
@@ -1,4 +1,5 @@
 openpower_update_manager_SOURCES += \
 	%reldir%/activation_ubi.cpp \
 	%reldir%/item_updater_ubi.cpp \
+	%reldir%/serialize.cpp \
 	%reldir%/watch.cpp