Refactor: Split Activation into common and ubi

Activations has a few functions coupled with ubi, split them into
ubi/activation_ubi

* Keep common code in activation.
* Make start/finishActivation() pure virtual.
* Move ubi specific code into ubi.
* Move ubiVolumnCreated into ubi.
* Make validateSignature() not inline, otherwise it gets compile error.

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

Change-Id: I7c8a0de6b0be4b1e9814ea75fd802a014b4aacfc
Signed-off-by: Lei YU <mine260309@gmail.com>
diff --git a/ubi/Makefile.am.include b/ubi/Makefile.am.include
index a836661..c75293d 100644
--- a/ubi/Makefile.am.include
+++ b/ubi/Makefile.am.include
@@ -1,3 +1,4 @@
 openpower_update_manager_SOURCES += \
+	%reldir%/activation_ubi.cpp \
 	%reldir%/item_updater_ubi.cpp \
 	%reldir%/watch.cpp