Clean up boot image generation

* Remove duplicate DTB generation step.
* Don't do unnecessary DTB concatenation step.

Change-Id: Ie2028b15a79b03fbd423107e8fb91e3e45a1998e
Merge branch 'cm-11.0' into kk_mkt

Conflicts:
	custombootimg.mk

Change-Id: I417511d006f5a48229ca52ed01958b0b5809b4f6