Fix build, TARGET_CPU_VARIANT must be set

Change-Id: I78f372e22a16d681aeb7ddea2cfb7682fc194ab2
Do custom boot.img without BOARD_CUSTOM_BOOTIMG_MK

This is useful until the custom boot img patch is merged
upstream.

Change-Id: Ib1a20cbb4a48d04d90394ae338c9ba3829e000db
Fix README

The local manifest example was wrong since lt26 has
been removed from AOSP.

Change-Id: Id128aa47e9f9cb3645d0aff90fdb9eb66bd42a0c