rhine: 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: I92598fe1771e979c1f6857d5ee634353bd14df53